The synthetic opiate loperamide (LPMD) is an antidiarrheal compound which affects both intestinal motility and the transport of water and electrolytes. We have investigated its effects on ion transport in the stripped ileal mucosa of rabbits, mounted in Ussing chambers in vitro. Addition of LPMD to serosal bathing medium resulted in a dose-dependent decrement of short-circuit current (Isc). LPMD (50 microM) significantly increased net Cl absorption (JClnet) and JRnet (assumed to represent HCO3 secretion). LPMD also inhibited the Isc increments evoked either by agents that increase 3',5'-cyclic adenosine monophosphate (cAMP) content (theophylline and prostaglandin E2) or by the Ca2+ ionophore A23187; the opiate, however, did not prevent the Isc change induced by 3',5'-cyclic guanosine monophosphate (cGMP)-related agents such as 8-Br-cGMP and Escherichia coli heat-stable enterotoxin. LPMD did not alter basal or secretagogue-stimulated tissue cAMP. In contrast, LPMD caused a small increase in cGMP content of stripped ileal mucosa, but not in that of isolated enterocytes. The role of Ca2+ in LPMD action is suggested by the significantly different effect of the drug on JClnet in the presence and in the absence of Ca2+ in the serosal solution.