Twenty-one men with active duodenal ulcer underwent hourly gastric acid and 4-hourly plasma collections under fasting conditions. A statistically significant circadian rhythm was demonstrated for the group by population-mean cosinor analysis, a statistical technique designed for time-dependent measurements, for the 24-hr acid secretory output but not for plasma gastrin concentration. These findings are compatible with the interpretation that the circadian rhythm of unstimulated gastric acid secretion, here observed in most patients of the active ulcer group and previously reported for a group of healthy subjects, does not depend critically upon a circadian-rhythm change in plasma gastrin. Chronobiologic statistical techniques add an important quantitative element to the time-dependent measurement of gastrointestinal function of which one example is gastric acidity, with high rates occurring during the evening and low ones in early morning.