The urinaxy excretion of electrolytes, chlorides and 5-hydroxy-indolylacetic acid (5-HIAA) after administration of either reserpine or serotonin in dogs was closely similar in pattern: lower Na+ and Cl-, and higher K+ and 5-HIAA. Antidiuretic hormone (ADH) administration, on the other hand, resulted in a different pattern: higher Na+ and Cl-, and lower K+ and 5-HIAA. This suggests that the antidiuretic action of reserpine is due to release of serotonin, depleted from body stores, and not to release of ADH.