Watery diarrhoea as distinct from dysentery is occasionally seen in intestinal amoebiasis, suggesting a component of intestinal secretion. To study the pathogenesis of this watery diarrhoea, we evaluated the effect of lysates of Entamoeba histolytica on active intestinal electrolyte transport using rabbit ileum and rat colon studied by the Ussing chamber-voltage clamp technique. Amoebic lysates added to the ileal and colonic mucosal surfaces did not alter electrolyte transport; in contrast, addition to the ideal and colonic serosal surfaces caused an increase in short-circuit current which was transient in the ileum but more prolonged in the colon. This increase in current corresponded to inhibition of active Na+ and Cl- absorption and apparent stimulation of Cl- secretion in rat colon. In rabbit ileum, the short-circuit current response was: (1) dependent on serosal Ca2+, (2) inhibited by serosally applied verapamil, (3) associated with reversible desensitization, and (4) only partially inhibited by heating. These characteristics were similar to those of a series of neurohumoral substances present in mammalian intestinal mucosa which affect active electrolyte transport by increasing the permeability of the basolateral membrane to Ca2+. Substances with these properties identified include serotonin, substance P and neurotensin. All three substances were shown to be present in amoebic lysates by radioimmunoassay. Serotonin was also present by high performance liquid chromatography (HPLC) and thin layer chromatography, and neurotensin by HPLC. Prostaglandins were not present by radioimmunoassay. Attempts were made to determine which of these neurohumoral substances contributed to the changes in intestinal transport caused by amoebic lysates. Serotonin was thought to be involved, from the inhibition of the transport effect of amoebic lysate on rat colon by anti-serotonin antibody and by bufotenine, which inhibits the effect of serotonin on ion transport. Prostaglandins also appeared to be involved, since pretreatment with PGE2 or indomethacin inhibited the effects of amoebic lysate on transport. We conclude that amoebae contain neurohumoral substances, including serotonin, neurotensin and substance P, which may be important in the intestinal secretion caused by amoebae. Serotonin appears partially responsible for the intestinal secretion. In addition, amoebae may induce prostaglandin synthesis by the intestinal mucosa which could also contribute to the secretory response. The relation between the neurohumoral substances which act by Ca2+ and the prostaglandins presumably caused to be synthesized in the intestinal mucosa is not known.