A cetazolamide, an inhibitor of the enzyme carbonic anhydrase, increased the urinary excretion of cyclic AMP in normal and parathyroidectomized rats. The increase was greater in rats with intact parathyroid glands than in parathyroidectomized rats. This rise in the urinary excretion of cyclic AMP was not due to an increase in urine flow or a change in urine pH. Furosemide caused an increase in urine flow, but did not affect the excretion of cyclic AMP or phosphate. Alkalinization of the urine with bicarbonate did not increase the urinary excretion of phosphate or cyclic AMP. Acetazolamide increased the productionof cyclic AMP by rat renal cortical slices in vitro. This effect was dose-dependent. Acetazolamide also stimulated the activity of renal cortical adenyl cyclase in a dose-dependent manner but had no effect on the activity of cyclic nucleotide phosphodiesterase. The pattern of urinary excretion of cyclic AMP and phosphate after administration of acetazolamide was similar to that observed in rats given parathyroid hormone. It is suggested that acetazolamide stimulates the renal production of cyclic AMP by activating adenyl cyclase and that this may be the mechanism by which this inhibitor of carbonic anhydrase produces phosphaturia.