The effect of two oral doses (10 and 20 mg) of nifedipine versus placebo on the fasted gallbladder volume and on the meal-induced gallbladder emptying was assessed according to a double-blind study protocol in 12 healthy volunteers. Eight subjects underwent three studies (with placebo and with both nifedipine doses), whereas in two subjects the effect of a 10-mg nifedipine dose vs placebo and in two others the effect of a 20-mg nifedipine dose vs placebo was examined. The studies were performed on separate days, and the gallbladder volume was measured by means of real-time ultrasonography. Neither placebo nor 20 mg nifedipine per os elicited any significant change in the fasted gallbladder volume. With 10 mg nifedipine per os a significant increase in the interdigestive gallbladder volume was observed: 22.9 +/- 2.9 cm3 before and 26.2 +/- 3.2 cm3 after the drug receipt (P less than 0.005). A trend towards an inhibition of the postprandial gallbladder emptying was observed with 10 mg nifedipine per os without, however, reaching the level of statistical significance. Following 20 mg nifedipine per os, a marked delay in the meal-stimulated gallbladder emptying occurred, as reflected by a decrease in the gallbladder ejection fraction from 48.1 +/- 4.5% (placebo) to 26.4 +/- 5.0% (nifedipine) (P less than 0.02) at 30 min and from 54.0 +/- 3.6% (placebo) to 33.2 +/- 4.6% (nifedipine) (P less than 0.02) at 40 min after the test meal. We conclude that a therapeutic oral dosage of nifedipine has a significant relaxing effect on the human gallbladder.