To investigate the mechanism responsible for the circadian variation of elimination processes of ethanol (EtOH), EtOH (100mg/100g B.W.) was orally administered to female Sprague-Dawley rats at 8: 00 and 20: 00 under non-fasting conditions, and EtOH concentration in plasma prepared from tail venous blood was determined with gaschromatograph at constant intervals. The intakes of feed and water 12 hours before oral administration of EtOH were also measured to elucidate the relationship between feed intake or water intake and each parameter obtained from the time course of plasma EtOH concentration (p [EtOH]). Immediately after EtOH was disappeared in Plasma, the rats were decapitated. Concentration of EtOH in stomach and the content in stomach for each rat were measured, and specific activity of alcohol dehydrogenase (ADH) in liver was determined spectrophotometrically at 37°C and pH 9. Solid feed and water were given to rats ad libitum during the light period (8: 00-20: 00) and the dark period (20: 00-8: 00). The feed intakes from 20: 00 to 8: 00 and from 8: 00 to 20: 00 were 5.7±1.6mg·g^-1 B.W. ·hr^-1 and 1.6±0.7mg·g^-1 B.W. ·hr^-1, respectively. The former was bigger than the latter. The water intake during the dark period was 2.2 times bigger than that during the light period. Therefore, the feed and water intakes during the dark period were higher than those during the light period. The max. p [EtOH] (A), the time to reach max. p [EtOH] (B), the time of pEtOH disappearance (C) and integration value of pEtOH (D) obtained from the dark period were 2.0, 1.5, 1.6 and 3.2 times bigger than those obtained from the light period, respectively. This means that there is the diurnal rhythm of the time course of p [EtOH]. Liver ADH activity showed nonsignificant change between the light period and the dark period. The remaining rates of EtOH in the stomach (= (Content of EtOH in stomach/total content of EtOH administered) ×100) were 25.8±10.7% for the light period, and 0.3±0.4% for the dark period, the former was 86 times higher than the latter. Therefore, A, B, C and D showed the diurnal variation under the non-fasting conditions. However, apparent decay rate constant (k) of EtOH showed no diurnal variation in both groups. There was bigh negative correlation (γ≅ -0.8) between each parameter (A, B, C and D) except k value and the remaining rate of EtOH in stomach. From these results, it is likely that the diurnal rhythm of p [EtOH] under non-fasting conditions is controlled by the remaining rate of EtOH in stomach and independent of the change of liver ADH activity.