Foveolar hyperplastic changes have been recently considered to be the most distinctive histological finding after partial gastrectomy, and their relationship with bile reflux has been proven. The present study was undertaken to establish whether an increase in 3H-thymidine uptake--which is an expression of increased gastric mucosal proliferative activity--might correspond to these hyperplastic changes. Histology and thymidine uptake values were assessed and compared in nineteen rats operated upon either by means of a Polya gastrectomy (11), or with a Roux-en-Y biliary diversion (8). Thymidine uptake values and the extent of gastric hyperplastic changes were greater in the Polya than in the Roux-en-Y group. Moreover, a significant correlation was found between thymidine uptake values and the extent of hyperplastic changes. The relationship between hyperplastic changes and post-gastrectomy increased mucosal proliferative activity would thus seem to have been demonstrated. Since bile acids have been shown to stimulate cell proliferation in ileal and colonic mucosa, a rôle for bile reflux can be hypothesized for the increased cell proliferation activity in the gastric stump.