The effect of testosterone (DT) and thyroxin (L-T4) on erythropoiesis and erythropoietin (Ep) production was studied in control and nephrectomized sheep fetuses beginning at about 100 d of gestation. Fetuses were given injections of either 1.2 mg/d x 13 of L-T4, 12 mg, once every 5 d x 3 of DT or the vehicle alone. Fetal plasma samples for Ep determinations were obtained before and at intervals after the start of the treatment. Reticulocyte and hematocrit levels, and the percent erythrocyte-59Fe uptake values were used to assess erythropoiesis in each fetus. No Ep was detected in plasmas of control fetuses, while significant amounts of Ep were present in plasma obtained from DT- and L-T4-treated intact fetuses. Bilateral nephrectomy did not diminish the Ep response to DT and L-T4. In both intact and nephrectomized fetuses, treatment with DT resulted in the production of significantly greater amounts of Ep than L-T4. The rise in Ep in all groups was accompanied by increases in reticulocytes (2.2 +/- 0.2% vs L-T4:8.1 +/- 0.4% and DT:7.6 +/- 0.7%), percent erythrocyte-59Fe uptake (20.5 +/- 2.9% vs. L-T4:36.7 +/- 3.8% and DT:39.1 +/- 4.0%) and hematocrit (31.2 +/- 2% vs. L-T4:41.8 +/- 3% and DT:48.6 +/- 4.2%). The enhanced erythropoiesis in all groups of nephrectomized fetuses was dependent upon the presence of Ep, because the administration of anti-Ep to these fetuses resulted in the suppression of erythropoiesis in all three groups. These data demonstrate that (a) DT and L-T4 are effective promoters of extrarenal Ep production, thereby enhancing erythropoiesis in intact and nephrectomized fetuses; (b) DT is a stronger stimulus of extrarenal Ep formation than L-T4; and (c) Ep is required for the expression of the erythropoietic effects of L-T4 and DT.