1. Biopsies of colonic mucosa from patients with ulcerative colitis liberated more interleukin-1β, prostaglandin E 2 , leukotriene C 4 and platelet-activating factor into the medium in which they were cultured than biopsies from patients with irritable bowel syndrome and histologically normal mucosa. 2. Addition of interleukin-1 stimulated release of greater quantities of all these inflammatory mediators, including interleukin-1 itself, from inflamed and normal mucosa. 3. Blockade of cyclo-oxygenase with indomethacin or of lipoxygenase with ICI 207968 or of phospholipase A 2 with mepacrine inhibited release of prostaglandin E 2 or leukotriene C 4 or both of these plus platelet-activating factor, respectively. 4. Interleukin-1 stimulated the short-circuit current across isolated rat colonic mucosa mounted in flux chambers in a dose-dependent manner ( K m 2 × 10 −11 mol/l). This stimulation was markedly inhibited by the removal of chloride from the bathing media. 5. Indomethacin or ICI 207968 inhibited the short-circuit current response to interleukin-1 and a combination of these antagonists produced a greater inhibition. Mepacrine caused an even greater inhibition whereas tetrodotoxin plus mepacrine inhibited the current completely. 6. These data indicate that interleukin-1, released in excess from inflamed colonic mucosa, stimulates the release of a range of inflammatory mediators as well as of more interleukin-1. It probably acts by stimulating phospholipase A 2 in inflammatory cells, probably lymphocytes, and can do so in normal and inflamed mucosa. Since, in rat colonic mucosa it stimulated an electrical response in very low concentrations, it is feasible that it is involved in the chloride secretion, and hence the diarrhoea, which may occur in inflammatory reactions. Hence treatment with mepacrine seems a prospect worth pursuing.