This report presents original methods to assess the bioavailability of an antifungal drug from a varnish preparation in finger nails. For the studies with human volunteers a ciclopirox 8 % nail lacquer was used to determine its efficacy in the treatment of onychomycoses. In vivo studies were performed on the fingernails of healthy volunteers by determining the total amount of ciclopirox penetrated per milligram of nail and the partition of the drug in the plate of the nails (technically divided into four layers). Ciclopirox concentrations were evaluated by measuring the inhibition of Candida pseudotropicalis growth in vitro. The ciclopirox concentration after 30 days treatment was determined as 3.35 ± 0.82 μg/mg nail material. This is a sufficient amount to kill the fungal pathogens. In addition, in vitro penetration experiments were carried out with excised pig skin. Lacquer formulations from 0.5 to 8 % were used to inhibit the growth of Trichophyton mentagrophytes. Formulations from 2 to 8% led to a strong to total inhibition of the dermatophyte after 30 min treatment time.