Myelinogenesis in developing rats was studied following chronic dosing with triethyl tin (TET), at a level of 1.0 mg TET/kg body wt/day. Experiments included starved controls with body weights depressed by 17 to 40% to equal those of the TET-treated groups. Rats at ages of 16, 21, and 30 days showed decreases relative to well-nourished controls in body weight, forebrain weight, myelin yield, cerebroside level, and specific activity of brain 2′,3′-cyclic nucleotide-3′-phosphohydrolase when dosed with TET. At 30 days, myelin and cerebroside yields were reduced by approximately 55%, while CNP activity was reduced by less than 20%. No differences in the forebrain myelin protein composition between control, starved, and TET animals were noted. The rate of myelin protein synthesis relative to brain total protein (assayed by incorporation of intracranially injected [3H]glycine into brain homogenate and myelin proteins) was decreased in the TET rats in proportion to the decreased yield of myelin, but no particular myelin protein was preferentially affected. Matching starved controls exhibited similar body weight decreases, less pronounced forebrain weight decreases, and little or no decrease in myelin concentration. There was a relative increase in the myelin protein synthesis rate in the starved rats, indicating preferential utilization of limited protein precursors for myelin protein synthesis. Spinal cord myelin was also decreased in the TET rats, but less severely than in the forebrain. At all ages optic, but not sciatic, nerves showed decreases in myelin concentration with TET treatment. We conclude that TET inhibits forebrain growth and CNS myelination more severely than can be accounted for by a general metabolic insult.