The pharmacokinetics of theophylline after both intravenous and oral administration was investigated in six hospitalized patients with normal renal, hepatic and pulmonary functions. A rather wide range of biological half-lives from about 3-16 hours and plasma clearance values of about 1.5-115 ml kg-1 hr-1 were found in the investigated patients, who were from 31 to 73 years of age. The apparent volumes of distribution during the eliminatory beta-phase (Vdbeta) were within the range 0.394-0.6161 kg-1 with a mean value of 0.484 1 kg-1 +/- 0.082 S.D., as determined from the intravenous data, and in excellent agreement with the value obtained from the peroral data. Except in one case theophylline exhibited two compartment characteristics after intravenous administration, while the oral data in only one patient showed this pharmacokinetic configuration and had to be analysed according to one-compartment characteristics in the other five subjects. In the oral experiments absorption rate constants of from about 0.57 to 2.17 hr-1 were found for the administered microparticulate theophylline tablet preparation, Nuelin from Riker Laboratories. A wide range of lag-times from 0 to 1.32 hours were also demonstrated in the experiments. The systemic availability of theophylline in this preparation varied from 82.8 to 103% as determined on basis of the ratios of areas under the oral and intravenous serum concentration curves. It is conclusively stated that therapeutic plasma concentrations of theophylline probably may be maintained and controlled efficiently with the investigated oral theophylline preparation. Because of the interindividual variability in the biological half-life of the compound monitoring of the serum theophylline concentration is generally advised in order to avoid toxic side effects, in particular in relation to the initial establishment of a therapeutic serum concentration level in the individual subjects to be treated.