Ninety normal volunteers were entered into a double-blind, placebo-controlled study to compare the efficacy of misoprostol (200 micrograms q.i.d.) vs. cimetidine (300 mg q.i.d.) in protecting the gastric and duodenal mucosa from tolmetin-induced (400 mg q.i.d.) injury. After 6 days of treatment, the degree of mucosal injury between treatments was compared by endoscopy, using a predetermined rating scale of 0 (normal mucosa) to 4+ (greater than 25 hemorrhages or erosions or an invasive ulcer). Utilizing a score of less than or equal to 2+ (2-10 hemorrhages or erosions) as a therapeutic success, the overall success rates were 8/30 (26.7%) for placebo, 19/30 (63.3%) for cimetidine, and 27/29 (93.1%) for misoprostol (p less than 0.001). Pairwise comparisons were also significant: misoprostol vs. placebo (p less than 0.001), misoprostol vs. cimetidine (p = 0.006), and cimetidine vs. placebo (p = 0.004). A separate analysis of the gastric scores alone revealed success rates identical to those in the overall evaluation; however, success rates in the duodenum for both misoprostol (29/29) and cimetidine (29/30) were extremely high and did not differ. It is concluded that misoprostol is highly effective and significantly better than cimetidine in protecting the gastric mucosa from tolmetin-induced injury; however, both agents were highly protective in the duodenum.