The intestinal inhibitory activity of 1-(3-cyano-3,3-diphenylpropyl)-4-phenyl-4-piperidine-carboxylic acid ethyl ester (diphenoxylate) after a single oral dose of 5 mg was studied in 10 volunteers with X-ray diagnostic methods. The diphenoxylate treatment lengthened the transit time of the barium sulphate to caecum from 3.2 to 4.3 h. The X-ray cinematography showed the slower duodenal emptying in 5/10 of the subjects, but only in 1 of these the velocity of the peristaltic wave was clearly decreased. The results demonstrate directly the acute inhibitory action of diphenoxylate on intestinal motility with the dose used generally 3--4 times daily for the symptomatic treatment of diarrhoea.