Pharmacokinetics and bioavailability of a new anhydrous sustained release theophylline preparation (GTR-80 300 mg, Theospirex) were compared to those of an orally administered marketed solution containing theophylline-ethylenediamine and to a sustained release preparation containing theophylline-ethylene-diamine (control preparation). Eight healthy volunteers received in a cross-over design GTR-80 (300 mg) and the control preparation (350 mg) at single doses - nine volunteers received the solution in a quantity corresponding to 200 mg anhydrous theophylline. During the observation period 0-24 h the relative bioavailability of GTR-80 (calculated on the basis of equivalent doses) was 135% versus control preparation and 80% versus solution. During the observation period 0-36 h, the comparison of the dose-adjusted areas under the serum level-time curves of GTR-80 showed 116% versus control preparation and 91.5% versus solution. Statistical evaluation revealed significant differences in the observation period 0-24 h. Extrapolation of the solution's serum values indicated a shift towards the non-significant range for the observation period 0-36 h. The protracted action of GTR-80 300 mg could be demonstrated by the significantly longer period elapsing before reaching serum peaks compared to that after dosing of the fast absorbing solution. The lower interindividual deviations with respect to the parameters of Cmax and AUC after administration of GTR-80 as against the control preparation should be of interest in therapeutical use.(ABSTRACT TRUNCATED AT 250 WORDS)