The absolute oral bioavailability of dapsone (DDS) was established in dogs and healthy volunteers by comparing AUC's after oral and intravenous administration. Five female inbred beagles each received 100 mg DDS orally and intravenously as a bolus injection in a randomized cross-over study. DDS serum concentrations were determined using HPLC. After both routes of administration, linear pharmacokinetics were observed, the elimination half-life (t1/2) amounting between 5.8 and 10.2 h. After oral administration, AUC values between 56.1 and 99.2 mg X h X l-1 were found, while after intravenous administration AUC's were between 63.5 and 98.1 mg X h X l-1. The absolute oral bioavailability, corrected for differences in t1/2, averaged 107 +/- 9% (SD). A similar study was carried out in 2 female and 3 male healthy volunteers. The intravenous dose was reduced to 50 mg and given as an infusion. Pharmacokinetics were linear after both routes of administration. The t1/2 values amounted between 15.6 and 30.4 h. AUC's ranged from 24.0 to 75.4 mg X h X l-1 after oral administration and from 13.3 to 37.5 mg X h X l-1 after intravenous infusion of half of the oral dose. The absolute, t1/2-corrected oral bioavailability was calculated to be 86 to 104%. Complete bioavailability of DDS was demonstrated in dogs and healthy volunteers. The method used in this study might help to detect possible DDS malabsorption in leprosy and dermatitis herpetiformis patients.