The role of sex hormones on parietal cell function and number was studied in the rat. Ovariectomy significantly increased parietal cell mass (from 18.26 +/- 0.16 x 10(6) to 24.14 +/- 0.89 x 10(6] as well as basal acid secretion (from 3.49 +/- 0.06 to 3.63 +/- 0.05 mu Eq/10 min). Castration decreased the number of parietal cells in the gastric mucosa of male rats from 25.57 +/- 0.18 x 10(6) to 18.65 +/- 0.12 x 10(6) and basal acid secretion (from 3.99 +/- 0.03 to 3.50 +/- 0.06 mu Eq/10 min). The results suggest that male and female sex hormones regulate basal acid secretion via their effects on the parietal cell.