The effect of the structure of organotin compounds on their toxicity and neurotoxicity to the developing rat has been studied. Oral administration was used after selection of a vehicle that gave uniform organotin solubilization as evidenced by toxicity and chemical solubility data. This vehicle was milk plus Tween-80. Eight different organotin compounds were systematically surveyed for effects in the neonatal rat. Trimethyl- and triethyltin were most toxic, and were somewhat more toxic than tri-n-propyltin. Tri-n-butyltin was somewhat less toxic, while tricyclohexyl-, triphenyl-, diethyl- and dimethyltin were least toxic. Some higher doses of trimethyl- or triethyltin caused neuropathological changes that were characteristic for each compound, these changes being absent for the other agents. Regardless of their toxicity, significant amounts of tin, as the element, were found in brain, kidney, and liver after treatment with all agents. However, detectable blood values were only obtained for trimethyl- and triethyltin.