The absolute bioavailability (f) of pirenzepine was determined in 27 intensive care patients receiving the drug for prophylaxis and therapy of upper gastrointestinal tract bleeding. A multiple oral and intravenous dosage regimen and the times of blood sampling were adapted to individual conditions and treatment. Mean f in the patients was 0.28, which was significantly higher than in 12 normal subjects (0.14). It showed no dependence on age (range 20-82 y), nor on the risk factors cardiac insufficiency, renal and hepatic dysfunction, gastrectomy (Billroth II) and bleeding gastrointestinal ulcers, nor on concomitant administration of metoclopramide or antacids. Due to the wide therapeutic index of pirenzepine, it is concluded that individualization of therapy is not necessary for patients in intensive care.