Forty pregnant cynomolgus macaques were treated daily from gestational day 20 to 50 by nasogastric intubation of 0, 25, 150, or 300 μg selenium as L-selenomethionine/kg body weight. In each group, 7–8 pregnancies were terminated by hysterotomy at gestational day 100 ± 2 and the fetuses were examined, while 2–3 pregnancies in each group were allowed to proceed to term. Selenium and soluble glutathione peroxidase were measured in: maternal, neonatal, and fetal plasma and erythrocytes; fetal kidney, liver, muscle, and placenta; and maternal breast milk. The area under the multidose maternal plasma selenium concentration:time curve, the maximum maternal plasma selenium concentration, and the maternal urinary selenium excertion rates were proportional to the L-selenomethionine dose. Selenium concentrations in all fetal and neonatal tissues were also proportional to maternal L-selenomethionine dose. Glutathione peroxidase was affected only in maternal erthrocytes, fetal kidney, and neonatal plasma. The selenium concentration in fetal plasma was an average 33% of that in maternal plasma. Although selenium concentration in macaque milk were doubled by the highest does, intrauterine selenium accumulation accounted for the majority of the neonatal selenium body burden. Despite the elevated selenium concentrations in fetal tissues, neonatal blood, and milk, no deleterious effects on neonates were observed. There results suggest that primate fetuses are well protected against selenium toxicity arising from high maternal L-selenomethionine intakes. © 1994 Wiley-Liss, Inc.