The present paper reports that nitric oxide (NO) released by sodium nitroprusside (SNP) is a potent activator of rat pineal cyclic GMP production without affecting cyclic AMP synthesis. Other drugs such as isoproterenol, vasoactive intestinal peptide, and peptide histidine isoleucine were ineffective in stimulating cyclic GMP production, but activated cyclic AMP production. However, L-arginine, the physiological precursor of NO, did not activate either cyclic GMP or NO synthesis. Because L-arginine failed to activate cyclic GMP production, results suggest that NO is not produced in the pineal gland, but behaves as a potent regulator of this cyclic nucleotide.