In a double-blind crossover study hyoscine-N-butylbromide (HBB) was compared with placebo in its inhibitory effect on duodenal motility stimulated by metoclopramide. Measurements were made both by the balloon technique and the open-tipped tube method with pressure transducer. 20 mg or 50 mg HBB or placebo were instilled in random sequence into the intestinal tract of 12 healthy volunteers. The frequency of Type II waves was determined in each experiment after administration of the test substance following prior metoclopramide stimulation. This frequency was used as the inhibitory index for each drug. The test substance was administered 15 minutes before a second stimulation. A four-way analysis of variance (mixed model) was applied to the calculated number of waves per period. Subjects were required to respond equally to stimulation with increased motility on both trial days as evidenced by a minimal difference in the averages following the first stimulation on each trial day and before application of the test substance. The frequency of Type II waves after 20 mg and 50 mg HBB was reduced in a statistically significant manner in each case when compared with placebo. The probabilities of error were p less than 0-05 and p less than 0-025.