Oral doses of caffeine (10, 50 or 100 mg/kg) given to rats together with paracetamol (200 mg/kg p.o.) inhibited paracetamol absorp tion and thus lowered the serum concentrations of paracetamol. This caf feine action was dose-dependent and similar to that on blood alcohol levels, as recently published by us. Delayed stomach emptying is very proba bly the main cause for the diminished absorption of orally given drugs in the presence of caffeine. Paracetamol analgesia was not decreased by caffeine, which, given alone (50 and 100 mg/kg), increased the pain threshold in rats. Only the lowest dose of caffeine (10 mg/kg), being itself ineffective, reduced paracetamol analgesia significantly.