Circulating erythropoietin (EPO) concentration increases during human and rat gestation, thereby contributing to the expansion of red cell mass. However, the mechanism(s) underlying gestational increases of the hormone is unknown. Our objective was to define whether the elevated EPO levels are secondary to decreased metabolic clearance or to enhanced production. The half-life of the hormone was also measured. A bolus and a constant infusion of 125I-labeled recombinant human EPO (125I-rhEPO) were administered to chronically instrumented conscious pregnant and virgin rats. The metabolic clearance rate of the 125I-rhEPO was slightly but significantly higher in gravid rats than in the virgin control animals (0.13 +/- 0.01 vs. 0.10 +/- 0.01 ml/min). The plasma half-life of 125I-rhEPO was 2.9 +/- 0.1 h for the pregnant rats and 2.9 +/- 0.2 h for the virgin controls. To confirm these results obtained by using 125I-rhEPO, EPO-rich plasma was generated in anemic rats and administered to another group of conscious virgin and pregnant rats. The half-life of homologous EPO was 2.9 +/- 0.5 and 3.3 +/- 0.1 h for gravid and virgin rats, respectively (P = NS). We conclude that elevated circulating EPO in rat gestation is secondary to increased biosynthesis and not to decreased metabolic clearance.