Concentrations of macaque chorionic gonadotropin (mCG) in placenta, blood and urine of rhesus monkeys have been measured by both radioimmunoassay and bioassay throughout gestation. mCG was easily detected and quantified in these specimens for a brief period in early pregnancy, but was not detectable between the 40th day of pregnancy and term in placental extracts, serum, or 40-fold urine concentrates. The apparent absence of mCG after the 40th day of pregnancy makes these macaques a valuable model for pregnancy research, where the absence of chorionic gonadotropin is experimentally desirable. Unlike women and some higher primates, the functional status of the fetal, placental and maternal endocrine compartments of macaques can be studied in the absence of circulating chorionic gonadotropin during mid and late gestation.