We tested the hypothesis that dioctyl sodium sulfosuccinate (DSS), an anionic detergent often described as a stool softener, blocks water absorption in the small and large intestines. Net water transport was measured under steady state conditions in vivo during single pass infusions of intestinal segments; each segment served as its own control. DSS, in concentrations which may obtain when it is fed in segments of rat jejunum, ileum, or colon. In man, 0.5 mM DSS inhibited water absorption by 80%. In rats, the degree of inhibition was linearly related to log10 of the concentration of DSS (0.1 to 2.0 mM). Furthermore, DSS altered the histological appearance of the surface absorptive cells of the rat colon. Because of these properties, DSS could be regarded as a stimulant laxative.