To examine the effect of glucagon in vivo on renal formation and excretion of cAMP, clearance studies were performed in patients with hypoparathyroidism and in parathyroidectomized rats. Four patients with idiopathic hypoparathyroidism and 2 patients with pseudohypoparathyroidism were studied during an intravenous glucagon infusion (20 micrograms/kg body weight). In all patients, glucagon induced a significant increase in nephrogenous cAMP and a 2- to 3-fold increase in fractional excretion of phosphate. The average increase in nephrogenous cAMP was from a baseline of 784 +/- 229 to 18,748 +/- 3,842 pmol/100 ml glomerular filtrate (GF) (p less than 0.01) and occurred 30-60 min after the beginning of the glucagon infusion. The effect of intravenous glucagon, given as a bolus, was further examined in parathyroidectomized rats. Glucagon elicited a significant increase in the urinary excretion of the nucleotide. The excreted cAMP was compared with its filtered load for each urine collection period. In the first two collections, 0-15 and 15-30 min, the filtered load of cAMP was higher than its urinary excretion. During the following periods, 30-90 min, the excreted urinary cAMP exceeded by far its filtered load, suggesting a net nephrogenous contribution to the excretion of the nucleotide. Infusion of exogenous cAMP to parathyroidectomized rats induced significant increments in the filtered load and urinary excretion of the nucleotide. Tubular secretion of extrarenal cAMP could not be detected during the cAMP infusion. These results provide evidence supporting in vivo a possible parathyroid-independent formation of nephrogenous cAMP after glucagon administration, in men and in rats. The glucagon-induced increase in nephrogenous cAMP seems to account, at least partly, for some of the renal actions of this hormone.