The gastrointestinal (GI) absorption in mice of the anticholinergic quaternary ammonium compound emepronium, assessed by the urinary excretion of radioactivity and unchanged drug, increased from 1% to 7% and from 0.5% to 5%, respectively, in the dose range 0.1 to 40 mg/kg, whereafter a plateau was reached. Changes in the metabolism were evident at doses exceeding 80 mg/kg when 80% of the radioactivity was excreted as unchanged emepronium, as compared to 50% at lower doses. No dose-dependent urinary excretion of radioactivity was detected following intravenous injection. The increase in the fraction of the dose excreted in the urine after oral administration therefore cannot be easily explained by a dose-dependent metabolism or renal excretion. However, a relationship was seen between the reduced gastrointestinal motility, produced by increasing doses of the drug, and the increased bioavailability. Atropine sulphate given subcutaneously (1-50 mg/kg) more than doubled the urinary recovery of a small pharmacologically inactive oral dose of emepronium (5 mg/kg) and thus confirmed the transit rate-dependent absorption of emepronium. The superimposed effect of the pharmacological action on the GI transit rate, derived from the inherent property of an anticholinergic drug, may be of special significance for a drug with a low degree of absorption and a high variability, such as has been observed in clinical trials with the quaternary drug emepronium (Cetiprin).