Three different sustained-release tablets of theophylline (Theo-Dur, Phyllocontin continus, Euphyllin Retard) and an oral elixir of aminophylline were administered to 12 healthy volunteers according to a crossover scheme. Plasma concentration of theophylline was monitored for 33 h after each administration using an HPLC reversed-phase method. The mean values and SD for total body clearance (0.054 +/- 0.010 l/[h X kg]), elimination half-life (6.0 +/- 1.2 h), and volume of distribution (0.455 +/- 0.046 l/kg) were calculated from the plasma concentration curves after the administration of elixir. The mean bioavailability of Theo-Dur was 94%, Phyllocontin continus 88%, and Euphyllin Retard 84%. The absorption was faster from Phyllocontin continus than from Theo-Dur or Euphyllin Retard. The time of peak concentration varied considerably after Euphyllin Retard but was less fluctuating among the subjects after Phyllocontin continus or Theo-Dur. In some subjects an extremely delayed peak (up to 24 h after administration) was observed after Euphyllin Retard.