We have previously reported that serum somatomedin concentrations are maintained in pregnant rats after hypophysectomy. Because rat chorionic somatomammotropin (rCS) might replace pituitary GH during pregnancy, the concentration of rCS was measured at intervals after hypophysectomy on day of pregnancy. By day 16 of pregnancy, the serum rCS of hypophysectomized (hypox) rats was actually higher than that of normal pregnant rats when measured by a lactogenic radioreceptor assay. Increased levels of lactogenic radioreceptor activity (L-RRA) were maintained in the serum of hypox rats throughout the remainder of pregnancy. The GH radioreceptor activity (GH-RRA) of serum of hypox pregnant rats was also greater than that of normal rats during the last days of pregnancy, but the activity was only about 1/20th that of the L-RRA with the assays employed. There was no significant difference between placental L-RRA and GH-RRA of normal and hypox pregnant rats. The difference in concentration could not be attributed to differences in the number of fetuses. We conclude that the high levels of rCS were sufficient to maintain serum somatomedin concentration in hypox pregnant rats. This effect of rCS could have been due to binding by GH or PRL receptors of the maternal liver.