In 11 volunteers gastric acid secretion was measured under basal conditions and after modified sham-feeding after 4½days' treatment with placebo tablets twice daily (placebo), pirenzepine, 50 mg twice daily (pirenzepine), benzilonium bromide, 17.5 mg twice daily (benzilonium 35), or benzilonium bromide, 35 mg twice daily (benzilonium 70), respectively. The first basal portion of gastric fluid was cultured aerobically and anaerobically, and its nitrite concentrations were measured by a colorimetric technique. Basal acid output was reduced 40% by pirenzepine, 71% by benzilonium 35, and 84% by benzilonium 70. Reduction of the stimulated acid output was 47%, 57%, and 74%, respectively. Mean bacterial count (in log10/ml gastric juice) after placebo was 3.50±0.81 (SEM). Only the treatment with benzilonium 70 gave significantly increased bacterial counts (6.41±0.68; p<0.01). Mean nitrite concentrations (inμmol/l) after placebo, pirenzepine, benzilonium 35, and benzilonium 70 were 2.90±1.26 (SEM). 3.90±1.17, 11.36±7.24, and 18.81±5.71, respectively. The last value was significantly different from that after placebo (p<0.025). Bacterial counts were negatively correlated to basal acid output (p<0.001) but not to stimulated acid output. Nitrite was directly correlated to bacterial counts and inversely correlated to basal and stimulated acid output. Even a short-lasting but strong inhibition of gastric acid output by antimuscarinics can change the intragastric milieu significantly. No significant changes occur after moderate reduction of gastric acid output.