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As deduced from the antidiuretic and hydro-osmotic activities, intracarotid injection of 10 µg melatonin into urethane-anaesthetized cats significantly decreased the pineal argininevasotocin (AVT) content and increased its level in cerebrospinal fluid (CSF) between 5 and 60 min after the injection. No AVT was detectable in the plasma of cats between 5 and 60 min after the intracarotid injection of melatonin. However, between 60 and 90 min, when AVT disappeared from the CSF, a significant amount of AVT was detectable in plasma. The intracarotid injection of 10 µg serotonin was unable to induce the release of AVT into CSF or plasma, or to affect the pineal AVT content. Although intracarotid injection of 10 ng melatonin was unable to induce the release of AVT during the daylight hours, the same amount of melatonin was highly potent in inducing the release of AVT into the CSF during the night in the dark. The sensitivity of AVT to melatonin appears to be induced by darkness because the same amount (10 ng) of melatonin was ineffective when injected into animals maintained with the light on during the night. Since it was necessary to administer 5 µg melatonin during the hours of daylight to match the effects of 10 ng melatonin given at night, it appears that melatonin is about 500 times more potent in inducing AVT release during the night in the dark. The present results further consolidate the concept that melatonin represents the releasing hormone for pineal AVT and at the same time strongly suggest that AVT is first released into the CSF and reaches the blood only secondarily after its absorption from the CSF.