Triethyltin (TET) has been shown to be neurotoxic when injected on postnatal day (PND) 5. In the present experiment the authors examined the toxicity of a single exposure to TET at several postnatal ages. Rat pups were injected ip with O(saline), 1.5, 3.0 or 6.0 mg/kg TET bromide on PND 1, 5, 10 or 15. In agreement with the previous data, PND-5 exposure to 6 mg/kg TET produced behavioral toxicity and decreased adult brain weight. High dose pups were less successful in decending on a rope at 20 and 21 days of age, and were hyperactive in figure-eight mazes at 29-30 and 57-58 days of age. The spatial distribution of activity was also altered: photocell counts were increased primarily in the figure-eight area of the maze. The size of the milk bands was reduced in 6 mg/kg pups injected on either PND 1 or PND 5. Preweanling growth was decreased following all injection ages; this reduction was most pronounced for pups exposed to TET on PND 1 and PND 5. Mating behavior was disrupted in 6 mg/kg males irrespective of age at exposure. These data demonstrate a differential sensitivity to the toxicity of TET during postnatal life, with maximal susceptibility on PND 5.