Sequential cyclosporine pharmacokinetic assessments were performed at four centres within the context of a double-blind, multicenter clinical trial comparing the safety and tolerability of the conventional formulation with cyclosporine for microemulsion in de novo renal transplant patients. The initial average daily oral dose was 9.3 mg/kg given in two divided doses every 12 hr with subsequent dose reductions individually titrated to maintain trough concentrations within the target therapeutic range. Pharmacokinetic profiles were assessed at week 2, once between weeks 4-6, and at week 12 in 12 patients on the conventional formulation and 9 patients on the microemulsion. Over the study duration, cyclosporine daily doses were comparable in both study arms and were reduced in parallel from 9.2 to 6.9 to 4.7 mg/kg/day at the three successive pharmacokinetic visits. Dose-normalized peak and area-under-the-curve (AUC) increased between the week 2 and week 4-6 assessments for both formulations. Thereafter, these parameters continued to increase for the conventional formulation but exhibited a high degree of within-in patient stability for the microemulsion between week 4-6 and week 12. Between-formulation comparisons indicated that the rate and extent of cyclosporine absorption from the microemulsion were significantly higher over the study duration. Specifically, at week 2, 4-6 and 12, dose-normalized AUC was 49%, 63%, and 32% higher for the microemulsion. Intrasubject coefficients of variability for pharmacokinetic parameters of the conventional formulation ranged from 26.3% to 68.2%. Corresponding values for the microemulsion were reduced by approximately half, ranging from 13.1% to 38.7%. The correlation between predose trough concentration and AUC was stronger for the microemulsion (r(2)0.819 vs. 0.635) over the full range of systemic exposures attained throughout the study. These results provide initial evidence that, as doses are reduced with time posttransplant, the cyclosporine dose-exposure relationship from the microemulsion may stabilize earlier than that from the conventional formulation, allowing increased pharmacokinetic control over cyclosporine use in this critical posttransplant period.