The effect of duodenogastric reflux on cell proliferation and mucosal mass in the stomach was studied. Male Wistar rats (n = 118) were submitted to Polya partial gastrectomy, partial gastrectomy with Roux-en-Y diversion of bile, total duodenogastric reflux or handling of the stomach alone (sham operation). Following oral administration of the carcinogen N-methyl-N'-nitro-N-nitrosoguanidine for 6 months, animals were killed 6, 9 or 12 months postoperatively and their stomachs were examined for crypt cell production rate and mucosal DNA content. Compared with shams, crypt cell production rate was more than twice as high in the gastric remnant 12 months after Polya partial gastrectomy (P less than 0.001) and median DNA content was 31 per cent greater (P = 0.05). After total duodenogastric reflux, DNA content was 62 per cent greater than in shams (P = 0.02), while Roux-en-Y diversion reduced crypt cell production rate by 65 per cent (P less than 0.001). Only Polya gastrectomy increased the number of rats developing gastric carcinomas (9 versus 2 shams; P less than 0.05). Increased mucosal cell proliferation in rats with duodenogastric reflux may help to explain the development of gastric stump cancer.