The development of adenylate cyclase responsiveness to vasopressin and parathyroid hormone was studied using membrane fractions prepared from medullo-papillary and cortical portions of kidneys of 2-46-day-old rats. The development of vasopressin binding capacity was followed on the same preparations, using [3H]vasopressin. The characteristics of medullo-papillary adenylate cyclase response to vasopressin were identical in young and adult control animals as regards apparent Km values for [Lys8]vasopressin (3 X 10(-8) M), specificity towards the natural neurohypophysial peptides and the effects of Mg2+. However, the magnitude of maximal enzyme activation by vasopressin was much lower in very young than adult animals. Accordingly vasopressin responsiveness increased sharply between the 10th and 25th days but the magnitude of the maximal response only reached the adult value between the 30th and 45th days after birth. During both periods basal adenylate cyclase activity was almost independent of age. Specific vasopressin binding sites were detected on kidney medullo-papillary membranes from young animals. Vasopressin binding capacity and adenylate cyclase responsiveness to the hormone followed similar development patterns. However, the appearance of specific binding sites slightly preceded the onset of adenylate cyclase responsiveness. Basal cortical adenylate cyclase activity/mg protein was 12 times higher in 2-day-old rats than in the adult controls. It dropped with age but only fell to the adult value between the 25th and the 35th days after birth. For the youngest animals tested (2 days old), the increase in activity due to parathyroid hormone was about half the increase measured in adults, and gradually rose to about 75% of the adult response between the 2nd and 46th days after birth. Apparent Km values for parathyroid hormone were identical in young and adult animals (3.2 and 3.0 U/ml, respectively).