Salmonella typhimurium, an organism that invades intestinal mucosa but does not elaborate a traditional enterotoxin, evokes ileal secretion by causing alterations in active sodium and chloride transport mechanisms. To evaluate the possibility that these changes in transport might be related to the adenylate cyclase-cyclic AMP or NA+-K+-adenosine triphosphatase (ATPase) systems, mucosal adenylate cyclase, cAMP phosphodiesterase, Na+-K+ and Mg++ ATPase activities, and cAMP concentrations were measured in rabbit ileal loops infected with two strains of S. typhimurium. Strain TML invades the mucosa and evokes fluid secretion whereas strain SL 1027 invades but does not evoke secretion. Cholera toxin-stimulated loops were also studied. When compared to control loops, TML-infected mucosa demonstrated a marked increase in adenylate cyclase activity, in cAMP concentration, and no change in phosphodiesterase or ATPase activities. SL 1027-infected mucosa demonstrated no change in either adenylate cyclase or ATPase activities. Indomethacin pretreatment of cyclase activation. In contrast, indomethacin pretreatment of cholera toxin exposed animals resulted in only a partial reduction of secretion while not altering the stimulation of adenylate cyclase. These results suggest that: (1) S. typhimurium causes ileal secretion by stimulating adenylate cyclase; (2) mucosal invasion alone (SL 1027) is not sufficient to activate adenylate cyclase, and (3) Na+-K+-ATPase does not appear to be involved in salmonella-induced secretion. The mechanism of salmonella activation of adenylate cyclase is unclear but apparently differs from that of cholera toxin in that it is inhibited by indomethacin. This might be explained by the participation of prostaglandins in the salmonella activation process.