Developmental effects of single prenatal trimethyltin exposure were examined in THA rats. Pregnant rats were injected on gestational day 12 with single dose of trimethyltin chloride (TMT) ip at either 0, 5, or 7 mg/kg. Significant differences between treated and control offspring rats could not be observed in terms of body weight, pinna detachment, incisor eruption, eye opening, surface righting, cliff avoidance, pivoting, negative geotaxis and auditory startle. Spontaneous motor activity and open field behavior in the rats were also not affected by TMT. In Sidman avoidance test, however, the avoidance rate of the treated offspring rats was lower when compared to that of the controls. These results suggest that prenatal TMT administration disrupts learning acquisition.