Tubules were isolated from kidneys of rats after incubation with collagenase and the effect of parathyroid hormone on adenyl cyclase in this tissue was studied. Adenyl cyclase in homogenates of tubules was assayed by measuring conversion of ATP- α 32P to cyclic 3′, 5′-AM32P. Incorporating an ATP-regenerating system (phosphoenolpyruvate and pyruvate kinase) enhanced the sensitivity of the system. Virtually all of the enzyme activity in the renal cortex was found in the tubules. Parathyroid hormone caused rapid activation of the enzyme in tubules from the renal cortex; enzyme activity was directly proportional to the concentration of parathyroid hormone over the range of 0.05 to 5 μ g/ml. The effect of parathyroid hormone was much greater with tubules isolated from the renal cortex than with those from the medulla. In contrast, arginine vasopressin activated the enzyme in the medulla but not the cortex. Epinephrine, glucagon and thyrocalcitonin caused an increase in enzyme activity but showed no dose-response relationship; thyrocalcitonin did not facilitate or inhibit stimulation by parathyroid hormone. Propranolol inhibited stimulation by epinephrine but phentolamine did not block activation by any hormone tested. The concentration of cyclic AMP in cortical tubules increased 2-fold after adding parathyroid hormone in vitro. These studies indicate that the renal site sensitive to parathyroid hormone is located in the tubules of the cortex where the primary effect of the hormone is mediated by activation of adenyl cyclase, with consequent increase in intracellular concentration of cyclic 3′, 5′-AMP.