Pirenzepine is a very hydrophilic compound. As a consequences its penetration through biomembranes and hence its absorption after oral administration is limited. The variability in plasma pirenzepine after a single oral dose of 50 mg (2 x 25 mg tablet) was investigated in ten countries in a total of 87 volunteers. Pirenzepine was measured by a specific radioimmunoassay. Plasma level profiles were similar in every country with a peak of about 50 ng/ml within two hours after the dose. The mean terminal half-life of disposition was 11 hours. Although the area under the plasma level curve was not normally distributed throughout the population, there was a narrow distribution around the median. It is often accepted that drugs or drug formulations with low bioavailability show high variations in the rates and extent of absorption. Pirenzepine, however, shows consistently uniform plasma level profiles, well within the therapeutic range after multiple dosing (50 mg, twice daily). Only a small proportion of the population may need individual adjustment of the conventional dose regimen.