Nine healthy subjects underwent two 24-h studies, either when fed six standard meals by mouth or when fasting. There was no significant difference in the median integrated 24-h intragastric acidity when fed or when fasting, 805 or 801 mmol.h/L, respectively. However, the median integrated 24-h plasma gastrin concentration was significantly higher when fed than when fasting, 284 pmol.h/L and 114 pmol.h/L, respectively (p less than 0.01). There appears to be a normal circadian rhythm for intragastric acidity: feeding appears to cause an acute decrease of intragastric acidity and to release gastrin, which in turn causes a compensatory rise of intragastric acidity. The results of this study suggest that the need to reinstate "normal" intragastric acidity is the drive to food-induced gastric acid secretion.