Cimetidine may protect gastric and duodenal mucosa by increasing cell turnover [5, 17]. Mucosal cell turnover in the duodenum of 7 control rats receiving intraperitoneal injections of normal saline for 6 days was compared with 8 rats receiving cimetidine for 6 days (500 mg/kg/24 h). Cell proliferation was assessed by autoradiography after injection of tritiated thymidine. The ‘leading edge’ was defined as the highest tritium-labelled nucleus in the crypt-villus (D/V) column expressed as a percentage of the complete C/V column. Ten C/V columns were measured in each rat. The number of tritium-labelled nuclei in 10 complete C/V columns was also counted in each rat. In the controls the median leading edge was 74.8% (25% quartile 70.6%, 75% quartile 76.2%) and in the rats receiving cimetidine 73.4% (25 % quartile 72.1 %, 75 % quartile 75.9%). The median labelled cell counts were 2,052 (25 % quartile 2,008, 75% quartile 2,173) and 1,999 (25% quartile 1,822, 75% quartile 2,041), respectively. These differences were not significant (p > 0.1; Mann-Whitney U test). Duodenal mucosal proliferation was assessed in 19 rats 48 h after two subcutaneous injections of 20 mg cysteamine and in another 9 after the same dose of cysteamine plus cimetidine (500 mg/kg/24 h). The median leading edge was 90.1 % (25 % quartile 84.4%, 75 % quartile 92.8 %) in cysteamine-treated rats and 76.7% (25 % quartile 71 %, 75 % quartile 79.9%) in rats receiving cysteamine plus cimetidine. The median cell counts were 3,354 (25 % quartile 3,031,75% quartile 3,738) and 1,975 (25% quartile 1,921, 75% quartile 1,980), respectively. These differences were significant (p < 0.01). This study has not shown increased cell turnover with cimetidine. Increased tritium labelling and increased values for the leading edge were found after cysteamine and cimetidine returned these to nor