The role of mood state (affect) and personality on basal acid secretion and basal serum gastrin concentrations were examined in seven healthy men and eight patients with duodenal ulcer. In each subject, gastric secretion and affect were assessed simultaneously on 5 separate days. None of 10 self-reported affect variables correlated with daily fluctuations in basal acid secretion in either group. Three variables (tension, conflict, and anxiety) correlated significantly with serum gastrin fluctuations in normal subjects, but these relationships were not present in patients with ulcer, who were hypergastrinemic regardless of their affective state. The degree to which serum gastrin fluctuated was unrelated to personality, as assessed by Minnesota Multiphasic Personality Inventory. On the other hand, several Minnesota Multiphasic Personality Inventory scales correlated with the degree of variability in basal acid secretion, including scales that measured impulsivity and social isolation/alienation. These studies indicate that serum gastrin concentrations are related to affective state in normal men, that this relationship is altered in men with duodenal ulcer, and that certain personality traits, such as impulsivity and social isolation, are associated with more labile basal acid secretion rates.