SummaryDuodenal alkaline phosphatase activities were measured in rats treated with cysteamine, a drug which causes duodenal ulcers in experimental animals. In the proximal duodenum, a significant decrease in alkaline phosphatase was observed 24 h after cysteamine injection. The inhibitory effect seems site-specific, since the enzyme in the upper jejunum remained unchanged. Moreover, the effect of cysteamine is apparently alkaline phosphatase-specific, because duodenal acid phosphatase, leucine aminopeptidase, and bicarbonate-dependent ATPase did not show any significant change in activity. A mechanism of the inhibitory effect of cysteamine on alkaline phosphatase is discussed.