The absolute concentrations of an antimycotic drug, econazole, 1-[2,4-dichloro-beta-(p-chlorobenzyloxy)-phenethyl]-imidazole nitrate, in the different layers of the skin are reported when the agent is applied to human skin in vitro and in vivo. The drug was tritium-labelled and incorporated into an ointment to give a 1% concentration as it is used in the therapy of dermatomycoses. This ointment was applied to the skin surface. After fixed time intervals, the skin was sliced parallel to the surface and the amount in each layer determined by liquid scintillation counting. In the in vivo investigations, the excretion of the drug in the urine was also determined. The results reveal that though up to 90% of the drug remain on the surface, about 20 mug/ml epidermal tissue and 1.5 mug/ml dermal tissue can be achieved. The amount which is excreted in the urine is equivalent to the quantity which enters the skin, i.e., the penetrating drug is absorbed by the circulatory system.