To investigate the pathogenesis of diarrhea in ulcerative colitis, colonic adenylate cyclase activity was determined in patients and normal subjects. Basal adenylate cyclase activity in 19 patients with active disease [61.5 +/- 9.6 (mean +/- SE) pmol cyclic adenosine monophosphate/mg protein . 10 min] was two times higher (p less than 0.01) than its activity in colonic mucosa of 30 normal subjects or 10 ulcerative colitis patients in remission [31.4 +/- 2.0 and 23.6 +/- 1.9 pmol cyclic adenosine monophosphate/mg protein . 10 min, respectively]. The enzyme activity was stimulated to the same extent in all groups by sodium fluoride, vasoactive intestinal polypeptide, or by substitution in the assay mixture of guanosine triphosphate by its hydrolysis-resistant analogue--GTP gamma S. Prostaglandin E2 significantly stimulated the enzyme activity in tissue obtained from normal subjects, patients with shigellosis, and ulcerative colitis patients in remission while it had no effect on adenylate cyclase activity in colonic mucosa of patients with active ulcerative colitis. These results suggest that stimulation of colonic adenylate cyclase activity, possibly secondary to the reported enhanced colonic prostanoid synthesis, may contribute to the diarrhea in ulcerative colitis.