The effect of different urea concentrations on the skin penetration of hydrocortisone from topically applied vehicle was investigated. Therefore 14C-labeled urea and 3H-labeled hydrocortisone were determined simultaneously in different layers of human skin. It could be shown an increased penetration of hydrocortisone is dependent above all on penetrated urea amount into the horny layer and on the distribution of penetrated urea in the horny layer. Hence follows a strong vehicle dependence from penetration promotion of hydrocortisone as an example for glucocorticoids by urea can be deduced. For the using of the penetration promoting effect of urea at the development of glucocorticoid containing ointments the knowledge of the glucocorticoid and urea penetration in each vehicle is indispensable.