The 30-min peak acid output (PAO30) to pentagastrin and insulin hypoglycemia has been compared in male and female ulcer patients. In male duodenal ulcer, prepyloric gastric ulcer and corporal gastric ulcer patients the PAO30 to pentagastrin exceeded that to insulin; in female duodenal ulcer patients the PAO30 to pentagastrin tended to be higher than to insulin but the difference was not significant. The PAO30 to both pentagastrin and insulin was higher in male than in female duodenal ulcer patients but the ratio of PAO30 for insulin and pentagastrin did not differ significantly which suggests that the sex difference in the responses is similar for insulin and pentagastrin. The ratio tended to be lower in the male corporal gastric ulcer than in the male duodenal ulcer patients which might indicate that insulin hypoglycemia is relatively less effective than pentagastrin in stimulating acid secretion in corporal gastric ulcer than in duodenal ulcer patients.