To elucidate the effects of alcohol on pineal rhythmicity, ethanol was administered in the evening in amounts usually consumed during social ingestion to nine healthy volunteers in a double blind, cross-over study. Plasma concentrations of melatonin, catecholamines (norepinephrine and epinephrine), and ethanol were measured by RIA, high pressure liquid chromatography, and gas chromatography before and for 12 h after the administration of 0, 0.5, and 1 g ethanol/kg wt. Plasma melatonin and catecholamines displayed expected diurnal rhythms, with peak values at 0300-0400 h for melatonin and trough values at 0100-0400 h for catecholamines. Intake of ethanol between 1900-1945 h inhibited the nocturnal melatonin secretion dose-dependently during the first half of the night, with no changes in urinary excretion of melatonin. The inhibition was 41% (P