1. The effect of pirenzepine, an antimuscarinic compound, on basal acid and pepsin secretion and on the kinetic characteristics (Vmax and ED50) of pentagastrin-stimulated gastric secretion was investigated in 11 duodenal ulcer male patients. 2. Each patient underwent two pentagastrin dose-response tests: one with placebo and the other with pirenzepine given as a 10-mg intravenous bolus followed by 2.5 mg/h continuous infusion. 3. Pirenzepine induced a marked reduction in basal acid secretion (4.4 vs 0.3 mEq/h) and pepsin secretion (76.3 vs 18.3 mPU/h). 4. The drug also caused a reduced response of parietal cells to pentagastrin, which resulted in an increase in ED50 (131 vs 299 ng kg-1 h-1). The maximal acid secretory response (Vmax) was reduced (40.9 vs 32.3 mEq/h), but this effect was not demonstrable when the result was expressed as total output minus basal output. 5. Pentagastrin-induced pepsin secretion was not significantly affected by pirenzepine. 6. We conclude that the inhibitory action of pirenzepine on gastric acid secretion results from the effect of the drug on basal secretion and on parietal cell responsiveness to stimuli.