HAPAB Bis(tri-n-butyltin) oxide ( TBTO ), the only effective substance in the commercial product Lastanox, was applied to the eyes of rabbits as Lastanox 'T' ( 20% TBTO in lower alcohols and non-ionic surface active substances and water ) and as Lastanox 'P' ( 15% TBTO in the same vehicle with the addition of bis(5-chloro-2-hydroxyphenyl)methane). Male and female albino rabbits, 10 months old, were divided into four experimental groups of six animals each and three control groups of four each. A single dose of 0.03 ml/head of Lastanox was applied in drops into the conjunctival sac of the left eye, once only, on the first experimental day. Group I received 10% Lastanox 'T' ( 6.1 mg/kg ); group II, 10% Lastanox 'P' ( 4.6 mg/kg ); group II, 1% Lastanox 'T' ( 0.61 mg/kg ); and IV, 1% Lastanox 'P' ( 0.46 mg/kg ). Groups V, VI and VII received by the same method a dose of 0.03 ml/head of the vehicle of Lastanox 'T', the vehicle of Lastanox 'P' and water, respectively. Results indicated that all the animals in the experimental groups were weak and showed hyperreflexia on the second to fifth days after application. These signs disappeared spontaneously by day 6 except in two rabbits, one each from groups I and II. These animals developed paresis of all limbs as well as respiratory difficulties and died on days 11 and 12. Lastanox 'T' and 'P' caused nearly identical changes in the eye and adnexa. Within 2 to 5 days after application, these changes in groups III and IV included eschars and ulcerations on the eyelids partly covered by pus. Between the eschars were numerous papules, pustules, hemorrhages and small necroses. The necrotic conjunctiva peeled off, leaving a bleeding underlying surface. Ulcerative surface erosions appeared on the cornea. The pupil was miotic, not rounded and showed no reflex. The iris was discolored and the pattern was absent. The red reflex coul/ not be seen. In groups I and II similar changes were observed, but they were considerably more pronounced both quantitatively and qualitatively. Macroscopic and microscopic findings are described in detail. The histoligical findings corroborated the clinical observations. In none of the control rabbits ( V, VI, VII ) were abnormal clinical, macroscopic or microscopic changes seen. Toxicology and Pharmacology 69/10/00, 350 1969