The effectiveness and safety of pirenzepine (PRZ) at 150 mg daily in the treatment of active duodenal ulcer was studied in an open pilot clinical trial followed by a double-blind trial against placebo. The open pilot study showed that pirenzepine was well tolerated and promoted ulcer healing in the 45% of patients within 2 weeks of treatment and in 90% within 4 weeks. The double-blind trial against placebo (PL) Confirmed these results: 60% of the patients in PRZ group and 10% of the patients in PL group showed endoscopic ulcer healing after 2 weeks. This difference was significant. Ninety per cent of the PRZ patients and 50% of the PL patients were completely healed after 4 weeks. In the PRZ group, gastric secretory tests showed a significant decrease in B.A.O. (69%), M.A.O. (33%) and P.A.O. (34%) after 2 weeks. In the PL group, the same parameters had only a small decrease, without statistical significance. There were no pathological changes in laboratory findings in either the open or the double-blind studies. Mild and transient side-effects were observed in 7 of 30 patinets receiving pirenzepine (diplopia and dryness of the mouth).