In excretion studies in dogs it was found that cyclic guanosine-3′∶5′-monophosphate (cGMP) was eliminated by glomerular filtration only. If there is any permeability of the tubule cells to exogenous cGMP, it must be very low. cGMP (5×10−5 moles) injected into the thoracic aorta of rats induced an increase of the renal electrolyte excretion. During the first 5 min after cGMP urine volune, sodium, potassium, and calcium excretion rates were elevated 2-to3-fold. There was also an increase of the glomerular filtration rate (GFR) but this was not statistically signifikant. In micropuncture experiments the intratubular administration of cGMP (10−2 M) into the proximal convoluted tubule delayed the half time (t 1/2) from 8.7 sec, (controls) to 13.8 sec, indicating a decrease of the tubular reabsorption capacity. We conclude that the diuretic action of cGMP is based on a decrease of tubular electrolyte reabsorption. The small increase of GFR may be an additional factor in the diuretic action of cGMP.