Ninety-seven consecutive patients with gastric surgery for peptic ulcer were studied; 86 had duodenal ulcer (DU), and 11 gastric ulcer (GU). DU patients were surgically treated by proximal vagotomy, proximal vagotomy and pyloroplasty, truncal vagotomy and pyloroplasty, or truncal vagotomy and antrectomy. All GU patients were operated on by the Billroth I method. Serum pepsinogen I(S-PG I), serum pepsinogen II (S-PG II), basal acid output (BAO), and maximal acid output (MAO) were determined before and 3 months and 1 year after the operation. The mean preoperative S-PG I concentration in DU patients (154 +/- 7 micrograms/l; mean +/- SE) was significantly higher than that (97 +/- 9 micrograms/l) in GU patients (p less than 0.001). A significant decrease in the mean S-PG I concentration in DU patients was seen 3 months (92 +/- 6 micrograms/l) and 1 year (66 +/- 4 micrograms/l) after the operation (p less than 0.001). This change did not depend on the type of vagotomy. However, this decrease was not seen in all individual patients as it was in BAO values. Moreover, the mean BAO decrease was much greater at 3 months (7% of the preoperative value) and 1 year (23%) after the operation than the respective decrease in S-PG I concentration. There was also no correlation between S-PG I and acid output (BAO and MAO) before and after the operation. In GU patients the decrease in mean S-PG I value after the Billroth I operation was smaller than in DU patients after vagotomy.(ABSTRACT TRUNCATED AT 250 WORDS)