The pharmacokinetic properties of hydromorphone in healthy young male subjects were studied after i.v., peroral, and rectal administration. After i.v. administration the following pharmacokinetic parameters were found: elimination half-life 2.36 +/- 0.5 h, hepatic extraction ratio 0.51, apparent volume of distribution 2.9 +/- 1.3 L kg-1 and volume of central compartment 0.23 +/- 0.2 L kg-1. The absolute bioavailability after peroral administration was 50.7 +/- 29.8 per cent, and that after rectal administration was 33 +/- 22 per cent.