The current study was carried out to evaluate pharmacokinetic profiles of viramidine and ribavirin in patients (n = 8 per dose group) with compensated hepatitis C infection following oral dosing of viramidine (400, 600, or 800 mg bid for 4 weeks). Pharmacokinetic parameters were determined on days 1 and 29 based on plasma, red blood cell, and urine concentrations of viramidine and ribavirin. The results indicate rapid absorption and conversion of viramidine to ribavirin after oral administration of viramidine. Viramidine and ribavirin exposure in plasma and RBCs generally increased from the 400- to 600-mg dose level of viramidine. However, no further increase in exposure was noted at the 800-mg dose. Long half-lives for viramidine (66-76 hours in plasma and 200-420 hours in red blood cells) and ribavirin (340-410 hours in plasma and 360-430 hours in red blood cells) were noted. A negligible amount of viramidine (1%-4% of dose) and a small amount of ribavirin (9%-14% of dose) were excreted in the urine. The renal clearance was low for both viramidine (5-8 L/h) and ribavirin (4-7 L/h). Significant accumulation of viramidine was noted in red blood cells (accumulation factor [R] = 5-8) but not in plasma (R = 2). Extensive accumulation of ribavirin was noted in both plasma (R = 9-17) and red blood cells (R = 77-129). Steady-state levels of ribavirin and viramidine in plasma and red blood cells were achieved by day 22. At steady state, there was extensive conversion of viramidine to ribavirin in both plasma and red blood cells. Both viramidine and ribavirin were preferentially distributed into red blood cells than plasma.