Misoprostol, a synthetic prostaglandin E1 analog, at doses of 200, 400, or 800 micrograms, and placebo were administered orally in random fashion to eight healthy male volunteers. The effects on basal and pentagastrin (0.6 microgram/kg/hr)-stimulated acid and mucus (N-acetylneuraminic acid measurement) secretion were then determined. Misoprostol in 200-, 400-, and 800-micrograms doses reduced basal acid secretion by 91%, 93%, and 93%, respectively. Mean 2-hr acid secretion was reduced by 27%, 33% (P less than 0.01), and 51% (P less than 0.01), respectively. Reductions in secretory volumes paralleled acid changes. Mucus secretion increased by 37%, 82%, and 95% during the basal period following misoprostol doses of 200, 400, and 800 micrograms, respectively. Increase in mucus of 27%, 31%, and 38% was observed during maximal acid inhibition (1-30 min) by misoprostol in 200-, 400-, and 800-micrograms doses. The concentration of gastric juice mucus was significantly increased. Subjects experienced no significant side effects during the study, and there were no significant changes in hematological or chemical blood studies. Misoprostol, a potent inhibitor of gastric acid secretion, also stimulates mucus secretion. This mucogenic effect may be important in the mucosal protective action of misoprostol and its antiulcer efficacy in man.