Postprandial hypotension during hemodialysis is a serious problem. The reduction of the blood volume (BV) by ultrafiltration and the redistribution of the blood from the large vessels to the splanchnic organs are regarded to be the principle mechanisms of postprandial hypotension during hemodialysis. The hematocrit (Ht) of the large vessels is expected to increase by ultrafiltration and also by peripheral shift of the blood, because Ht of the peripheral vessels is much less than Ht of the large vessels. To analyze the effects of food intake during hemodialysis on BV of the large vessels quantitatively, we monitored Ht of the arteriovenous shunt blood continuously in the patients treated with hemodialysis regularly and estimated BV using the following equation, i.e., BV/initial BV = initial Ht/Ht. The rate of ultrafiltration was kept constant during the study. The reduction rate of BV was expressed as percentage of the initial BV per hour (delta BV). Ultrafiltration rate was expressed as percentage of the dry weight per hour (delta BV), delta BV was 3.24 +/- 0.57%/h (mean +/- SE) before the meal and increased to 13.99 +/- 0.91%/h almost instantaneously when the meal started in the supine position. The minimum value of BV was less than that estimated from the ultrafiltration rate by 2.65 +/- 0.26% of the initial BV. The effects of food intake disappeared in 43 +/- 3 minutes. When the meal was taken in the sitting position, delta BV was 28.21 +/- 2.14%/h, and the minimum value of BV was less than that estimated from the ultrafiltration rate by 4.69 +/- 0.70% of the initial BV. We conclude that food intake during hemodialysis decreases BV of the large vessels transiently but significantly. The effects of food intake on BV were more severe when the meal was taken in sitting position than those when the meal was taken in supine position.