Because of the close similarity of their chemical structure, production methods and use pattern, the two isomers of butyl methacrylate, n-butyl methacrylate (n-BMA) and isobutyl methacrylate (i-BMA) are considered together in this report. The abbreviation n-BMA/i-BMA is used whenever the text is applicable to both isomers. At room temperature, n-BMA/i-BMA is a clear, colourless, flammable liquid with a faint, characteristic ester odour. Both compounds have a low solubility in water and are soluble in most organic solvents. In the EU, about 30 kt of n-BMA and 8 kt of i-BMA were produced in 1994. Environmental releases during production and major industrial uses are very low. When released into the environment, the majority (95 %) of n-BMA/i-BMA will partition into the atmosphere. The atmospheric half-life of n-BMA/i-BMA has been estimated to be between 5.6 and 7.5 hours depending on the parameters of the calculation. In water n-BMA/i-BMA is readily biodegradable. A moderate bioaccumulation potential is expected. A half-life of 13 hours for n-BMA and 5.6 hours for i-BMA has been calculated for volatilisation from a model river. In soil, n-BMA is characterised by moderate adsorption, while i-BMA was found to be strongly adsorbed. n-BMA/i-BMA is of moderate toxicity (EC50: 10-100 mg/l) to bacteria, fish and Daphnia. i-BMA had a high toxicity (0.1 mg/l) towards green algae when tested in a closed system based on a 96-h NOEC of 0.047 mg/l. However, this effect was reversible and represented an algistatic rather than an algicidal action of i-BMA. When tested in a standard open test system the toxicity of n-BMA to green algae could not be measured conclusively because of the rapid loss of the material during the exposure time. The test results of the algae studies indicate that the hazard identified by the growth inhibition observed with i-BMA in a closed system should be viewed in the light of the reversibility of the effect and the probable volatilisation under open exposure conditions which represent more closely the normal environmental situation. This accounts for the lower toxicity of n-BMA observed in an open test system. From the physico-chemical data it is anticipated that n-BMA/i-BMA is rapidly absorbed after oral or inhalation exposure. In vitro studies using isolated rat liver microsomes or porcine liver esterase showed rapid hydrolysis of n-BMA yielding methacrylic acid and n-butanol. No in vivo metabolism data are available on n-BMA/i-BMA, but from the in vitro data rapid hydrolysis to methacrylic acid and the corresponding alcohol can be anticipated. n-BMA did not bind to glutathione (GSH) in vitro. It is expected that after hydrolysis the respective cleavage products methacrylic acid and n-butanol or isobutanol are further metabolised by normal physiological pathways ultimately to CO2. The interpretation of the toxicological data is based on these assumptions. In mammals n-BMA/i-BMA is of low acute toxicity by the oral, dermal or inhalation route of exposure. They have local irritating properties to rabbit skin and eyes. Respiratory tract irritation was observed after inhalation exposure of rats to n-BMA. Although no data on respiratory irritation are available for i-BMA it is expected to have similar effects as n-BMA. Whilst n-BMA is a weak skin sensitiser in guinea pigs, there is no such evidence for i-BMA. From the available human clinical data it can be concluded that the sensitisation potential to humans of n-BMA is low. A repeat dose oral study in mice is available which, although of limited reliability, indicates that n-BMA is of low oral toxicity. A reliable 28-day exposure inhalation study in rats is available for n-BMA. The lead effect in this study was the formation of nasal lesions indicative of a local irritant effect in the olfactory region of the nose with no indications of systemic toxicity. Neither n-BMA nor i-BMA was mutagenic in a number of gene mutation assays with Salmonella typhimurium. i-BMA was not clastogenic in a mouse micronucleus assay. Despite the rather limited database, taking into account the close structural similarity of the two isomers and data on other methacrylic acid esters, there is no immediate concern for genotoxicity. For the endpoints carcinogenicity, chronic toxicity and toxicity to reproduction, no reliable data are available for either ester. However, information concerning potential hazards of n-BMA/i-BMA may in part be inferred from studies with methyl methacrylate which like n-BMA/i-BMA is metabolised to methacrylic acid in animals and in humans. Given the lack of carcinogenicity observed with methyl methacrylate and the lack of genotoxic potential of n-BMA/i-BMA, there is no immediate concern with regard to a possible carcinogenic potential of n-BMA/i-BMA. The Task Force is not aware of any reliable data on the carcinogenicity of n-butanol and isobutanol. With regard to chronic toxicity the lead effect for methyl methacrylate long term inhalation exposure was local nasal irritation. The lead effect in a 28-day n-BMA study is consistent with this pattern of toxicity and therefore suggests that this study is an appropriate and relevant indicator of chronic toxicity. Due to the paucity of data, no firm conclusions can be made on the potential toxicity to reproduction. However, the data available for methyl methacrylate and n-butanol and isobutanol suggest that there is no need for immediate concern for possible developmental effects arising from inhalation exposure to non-maternally toxic concentrations of n-BMA/i-BMA. Limited data available from repeated dose studies with n-BMA, methyl methacrylate, methacrylic acid and a fertility study with n-butanol did not reveal any indications for possible toxic effects on the reproductive organs. The sensitisation potential of n-BMA to humans is low. Despite the use of n-BMA/i-BMA for many years, no adverse systemic health effects have been observed. The lead effect of n-BMA/i-BMA identified in acute and subchronic animal studies is the local irritation at the site of contact following exposure by the inhalation route, the major route of occupational exposure. From the available studies it can be concluded that upper respiratory tract irritation is the most common effect of inhalation exposure to n-BMA/i-BMA.