Ketoconazole administered orally is used in the treatment of superficial and deep mycoses. To evaluate its active concentrations in skin tissue, serum, suction blister fluid (SBF), and cantharides blister fluid (CBF) levels of total and non-protein bound ketoconazole were determined. In general, only the free drug is considered to be the active one. Six healthy subjects received 200 mg once daily for 5 days. Total ketoconazole concentrations were determined by HPLC. The unbound fractions of ketoconazole in SBF (2.3%) and CBF (1.2%) were calculated from plasma protein binding (99.0%). Before the ultimate dose, levels of unbound ketoconazole in SBF and CBF were 0.64 +/- 0.16 and 0.70 +/- 0.25 ng/ml and were thus in accordance with free ketoconazole serum levels (0.52 +/- 0.24 ng/ml; p greater than 0.05). Furthermore, following the ultimate dose, the areas under the blister fluid level-time curves of unbound ketoconazole did not differ from the respective areas under the serum level time curves, thus distribution equilibrium between serum and skin blister fluid was obtained. Peak concentrations of free ketoconazole were (SBF) 8.6 +/- 2.9 ng/ml and (CBF) 8.9 +/- 2.3 ng/ml. Free concentrations in SBF and CBF were far below the MIC values for dermatophytes and Candida ssp. reported in the literature, leaving the concentration-effect relationship of ketoconazole still open for discussion.