Inhibition of ATPase activities by triethyltin (TET), diethyltin (DET), monoethyltin (MET) and trimethyltin (TMT) was studied in homogenates of brain and liver from adult rats. MET did not produce significant inhibition. ATPase activities in brain and liver homogenates from TET-treated adult rats did not differ from controls. Mitochondrial ATPase in brain homogenates from 5-day-old rats was two orders of magnitude more sensitive to TET than brain homogenates from adult rats. Isolated mitochondria and synaptosomal fractions from adult and neonatal brains were equally sensitive to TET. It is concluded that in vivo brain tin concentrations in 5-day-old rats following a neurotoxic dose of TET are sufficient to inhibit brain mitochondrial ATPase, whereas in adults, tin concentrations are insufficient for inhibition. In the adult rat, TET binding to myelin appears to prevent inhibition of brain mitochondrial ATPase, and the target of toxic action may be myelin.