We investigated the distribution of hydrolytic enzymes which metabolize prednisolone 21-acetate (PNA) to prednisolone (PN) in human skin. Km (Michaelis-Menten constant) and Vmax (maximum rate) of hydrolytic enzyme in human skin was 25.1 microM and 0.46 nmol/min/mg protein, respectively. Specific activities of hydrolysis in dermis and epidermis were similar and, in consideration to their thickness, hydrolytic activity in epidermis was 12.1 times higher than in dermis. Moreover, the highest amount of metabolite (PN) was found at 80-120 microm from the skin surface by skin slicing. Therefore, hydrolytic activity which metabolized PNA was distributed in epidermis, especially in the basement membrane area; epidermis borders dermis in this area and the papillary plexus is reached just beneath the dermal papillae. These results suggest that the distribution of hydrolytic activity in human skin may prevent certain substances from entering the systemic circulation in their unhydrolyzed form.