It is known, that Pirenzepine inhibits basal, pentagastrin-, insulin- and peptone-stimulated gastric secretion. In this study the effect of three different doses of Pirenzepine was studied on acid secretion stimulated by modified shamfeeding (MSF). Each of eleven healthy volunteers underwent four secretion tests after pre-treatment for 4 days in random order by: no drug, tablets Pirenzepine 25 mg b.i.d., 50 mg b.i.d., or 50 mg t.d.s. Basal acid output 0-30' was reduced by 48%, 59% and 66% respectively and stimulated acid output 0-120' by 45%, 58% and 48% respectively. When acid secretion was calculated as volume corrected for duodeno-gastric reflux and pyloric losses the corresponding figures were 33.7%, 36.3% and 42.6%. Inhibition of MSF-stimulated acid secretion by a high dose of Pirenzepine was not complete. Pirenzepine 50 mg b.i.d. seems to be a suitable dose for clinical use, as acid reduction is as good as with 50 mg t.d.s. and side-effects are known to be less.