This study was done to investigate aspects of control of extrarerial erythropoietin (Ep) production. To this end we studied the effects of three stimuli of renal Ep production in the adult, i.e. hypoxia, cobalt, and activation of adehylate cyclase on Ep generation by cultured fetal mouse liver cells. The fetal liver was taken as a model for extrarenal Ep production because this organ is considered the predominant site of extrarenal Ep production. We found that Ep production by the cells increased as the oxygen concentration was decreased in the incubation atmosphere from 20% to 1%. Cobalt (10-4–10-5 M) had no effect on Ep production. Activation of adenylate cyclase by forskolin (10-5 M) or isoproterenol (10-6 M) greatly enhanced Ep production. These findings indicate that the Ep-stirriulating effect of cobalt is specific for the kidney. However, oxygen depletion and activation of adenylate cyclase seem to be more general stimuli in Ep-producing cells. Furthermore we found that Ep production in hypoxia correlated with lactate formation in the cultured liver cells. This finding suggests that Ep production in fetal livers under hypoxic conditions parallels the shift from aerobic to anaerobic cellular energy metabolism.