The purpose of this study was to characterize the pharmacokinetics and determine the absolute bioavailability of 2'-deoxy-3'-oxa-4'-thiocytidine (dOTC) (BCH-10652), a novel nucleoside analogue reverse transcriptase inhibitor, in humans. dOTC belongs to the 4'-thio heterosubstituted class of compounds and is a 1:1 mixture of its two enantiomers, () and (+) dOTC. Twelve healthy adult male volunteers each received oral (800-mg) and intravenous (100-mg) doses of dOTC in two study periods separated by at least 7 days. Sixteen plasma samples were obtained over 72 h and assayed for () and (+) dOTC, and the resultant data fit by candidate pharmacokinetic models. Data were weighted by the fitted inverse of the observation variance; model discrimination was by AIC. The pharmacokinetic model was a linear, three compartment model, with absorption occurring during one to three first-order input phases, each following a fitted lag time. The model goodness-of-fit was excellent; r² ranged from 0.995 to 1.0. The mean absolute bioavailabilities of (+) and () dOTC were 77.2% (coefficient of variation [given as a percentage] [CV%], 14) and 80.7% (CV%, 15), respectively. The median steady-state volume of distribution for (+) dOTC, 74.7 (CV%, 19.2) liters/65 kg, was greater than that for () dOTC, 51.7 (CV%, 16.7) liters/65 kg (P < 0.05). The median total clearance of (+) dOTC was less than that of () dOTC, 11.7 (CV%, 17.3) versus 15.4 (CV%, 18.6) liters/h/65 kg, respectively (P < 0.05). The intersubject variability of these parameters was very low. The median terminal half-life of (+) dOTC was 18.0 (CV%, 31.5) h, significantly longer than the 6.8 (CV%, 69.9) h observed for () dOTC (P < 0.01). No serious adverse events were reported during the study. These results suggest that dOTC is well absorbed, widely distributed, and well tolerated. The terminal half-lives indicate that dosing intervals of 12 to 24 h would be reasonable.