The pharmacokinetics and bioavailability of theophylline have been investigated in eight healthy subjects following application of the drug in form of a tablet, a retard tablet (enteric-coated), and a suppository (Euphyllin) in comparison with an intravenous preparation. Peak plasma concentrations were measured 1.5 hours (tablet), 6 hours (retard tablet) and 4 hours (suppository) after drug administration, respectively. The absolute bioavailability was of th tablet 105 +/- 25%, of the retard table 81 +/- 23%, of the suppository 74 +/- 25%. The variations observed were primarily due to variations in the other pharmacokinetic parameters, only to a lesser extent to variable absorption of theophylline.