Fecal alkaline phosphatase excretion was evaluated as a marker for intestinal damage in rats. Animals received either intraperitoneal bleomycin or saline. Controls were pair-fed with animals in the bleomycin group throughout the study. Both groups demonstrated similar patterns of fecal alkaline phosphatase excretion. There was, however, marked daily variability of fecal enzymatic activity. Fecal alkaline phosphatase excretion was largely composed of intestinal alkaline phosphatase, which was characterized by enzymatic inhibition with L-phenylalanine. Dietary intake as well as daily fecal output and protein excretion were reduced immediately following bleomycin injections and gradually increased to baseline values by 7 days. It appeared that both the direct toxic effects of bleomycin and dietary intake influenced fecal alkaline phosphatase excretion. Routine clinical application of this assay may be limited because of the number of factors which may affect its excretion.