The pharmacokinetics and bioavailability of aciclovir (CAS 59277-89-3) were examined after administration of newly developed 200 mg and 400 mg tablets. Two studies, each with 24 subjects of either sex, were performed. In the three-way study I, two different tablets containing 200 mg of aciclovir (test and reference products) and a short infusion of 250 mg aciclovir were compared. In the two-way study II, the bioequivalence of a newly developed 400 mg aciclovir tablet was tested against a standard product. Irrespective of dose, the peak plasma aciclovir levels were obtained 1.5 h after administration of the tablets. In the case of the 200 mg tablets, the mean Cmax-values were 454 ng/ml (pilot test formulation, T) and 525 ng/ml (reference formulation, R), whereas the mean Cmax-values after administration of the 400 mg tablets were 779 (T) and 800 (R) ng/ml for test and reference formulation, respectively. The mean AUC calculated to the time of the last measurement in each instance was in the order of 2290 (T) and 2275 (R) ng/ml x h (200 mg tablets) or 3726 (T) and 3855 (R) ng/ml x h (400 mg tablets). The amount of the dose renally excreted as unchanged aciclovir was measured at 20.8 (T) - 21.8 (R)% with the 200 mg tablets and 14.3 (T) - 15.1% (R) with the 400 mg tablets.(ABSTRACT TRUNCATED AT 250 WORDS)