Diethylstilbestrol (DES) is considered a teratogen and a transplacental carcinogen in humans. In order to compare its biotransformation in nonhuman primates to that in humans, a metabolic study was carried out in rhesus monkeys and chimpanzees. After an oral dose of 1 mg/kg [14C]DES, approximately 59% of the ingested radioactivity was found in the urine and 28% in the feces of two female rhesus monkeys after 4 days, while in male rhesus monkeys urinary radioactivity accounted for 43% and fecal radioactivity for 35%. In chimpanzees, 63% of a 0.5 mg/kg dose was excreted with the urine in a female and 47% in a male animal. In both species, urinary radioactivity was predominantly (> 70%) associated with glucuronides. Besides DES, three metabolites were found in the urinary glucuronide fraction of rhesus monkeys and chimpanzees by radio gas chromatography and were identified as dienestrol and ω‐hydroxy derivatives of DES and dienestroi. Fecal radioactivity in rhesus monkeys was shown to consist exclusively of DES. Glucuronidation of DES appears to occur in neonatal and fetal rhesus monkeys.