The metabolic clearance rate (MCR) and production rate (PR) of circulating chorionic growth hormone-prolactin (CGP) were estimated from measurements of plasma concentrations of endogenous CGP following delivery of the placenta and of 131I-CGP following intravenous administration of the tracer. The MCR did not vary appreciably in the 3 pregnant and 4 nonpregnant subjects who were studied by the tracer method; the mean value was 173 1/day. At delivery, the PR estimated by this procedure was 1.03 g/day. Utilizing the rate of disappearance of endogenous CGP from plasma following delivery of the placenta in 6 pregnant women and a volume of distribution estimated from the tracer experiments, a mean PR of 1.78 g/day was found. Both the tracer and endogenous CGP disappearance curves could be resolved into at least 2 exponential components. The validity of the calculations of MCR and PR by these methods is discussed. The results of the studies of MCR in the light of other evidence suggest that the concentration of plasma CGP is directly proportional to PR, and the determination of plasma CGP in random samples of pregnancy blood provides a useful index of the production rate in contrast to estimations of the 24-hr urinary excretion. Only a small proportion of the total PR of CGP appears in the urine. Further support was obtained for the virtually unidirectional secretion of CGP into the maternal circulation. No evidence of sequestration of circulating CGP by the placenta was detected.