Hypophysectomized or sham-operated male rats were exposed to hypoxia (0.42--0.40 or 0.37--0.35 atm for 6, 12, or 24 hr) applied 2 wk to 7 mo after surgery. Erythropoietin (Ep) levels in rat serum were evaluated on the basis of the exhypoxic polycythemic mouse assay. Ep activity evoked by hypoxia was significantly lower in hypophysectomized rats than in sham-operated controls. Progressive increase of the EP response to hypoxia correlated with extension of the time interval between hypophysectomy and hypoxia from 2 wk to 2--4 mo apparently mediated by the simultaneous inverse decline of red cell mass (RCM) values, i.e., of the "relative plethora" induced by a low O2 demand associated with relatively high RCM values. However, after 3--7 mo hypoxic Ep activity was still lower than in sham-operated controls. In these ablated animals the relative plethora became negligible or absent; accordingly, the Ep response apparently had reached plateau levels. These studies indicate that hypophysis (hypophyseal and target hormones, with the exception of estrogens) modulates Ep production under hypoxic conditions, possibly via a permissive enhancement of renal Ep activity.