Wistar rats were injected just once, intraperitoneally with cortisol (1 microgram/g) or saline at the age of 5 days. The cortisol-treated rats did not differ significantly in the (U/P)osm ratio from the saline-treated controls before 15 days of life. Their response to ADH was distinct but weaker than in the saline controls aged 30 days. This reduced response persisted to 60 days of life. In the collecting tubule fragments, (3H)AVP specific binding was lower in the cortisol-treated rats than in the controls at the age of 20 and 60 days. There was no (3H)AVP specific binding in the proximal convoluted tubules in the cortisol- and saline-injected rats of both ages. The ontogenetic patterns of cAMP specific binding in the papillary cytosolic fraction were different: the early increase in cAMP binding was protracted in the cortisol-treated rats, and no peak appeared at the age of 25 days. Cytosolic protein kinase activity was lower, no peak appeared at 30 days, no activation of protein kinase occurred to the end of weaning in the cortisol-treated rats. The difference between the cortisol and saline groups was abolished by day 30. The interference of cortisol with the ontogenetic changes in AVP binding capacity and cAMP-dependent protein kinase appears to be a plausible cause of the altered development of the response to ADH.