Acetorphan is a potent enkephalinase inhibitor displaying antidiarrhoeal activity attributable to its intestinal antisecretory action mediated by endogenous enkephalins. The effect of acetorphan on digestive motility was studied in 12 healthy volunteers. Oro-caecal transit time was evaluated using the sulphasalazine/sulphapyridine method and colonic transit times using radiopaque markers. These measurements were successively performed after one week treatment with an antidiarrhoeal dose of acetorphan (100 mg t.d.s.) or placebo. There was no significant modification in transit time linked to acetorphan treatment: total oro-caecal times were 303 +/- 32 min vs. 287 +/- 27 min and colonic transit times 25.8 +/- 5.8 h vs. 31.3 +/- 5.5 h after acetorphan and placebo, respectively (means +/- S.E.M.). There was no significant modification either in right colonic, left colonic or rectosigmoid segmental transit times, or in the mean number of stools. These results, consistent with those from animal studies, confirm that, unlike classical antidiarrhoeal mu opiate receptor agonists, which act by delaying intestinal transit, acetorphan does not affect the transit. Antidiarrhoeal activity not accompanied by a delayed intestinal transit could have beneficial therapeutic consequences in the management of infectious diarrhoea. In addition, we show that the sulphasalazine and radiopaque markers methods can be simultaneously applied in the same study.