The penetration of corticosteroids through epidermal membranes in vitro proved to be better when the corticosteroid was dissolved in, or salicylic acid was added to, the vehicle. Hydrocortisone 17 butyrate (HCB) penetration from an ethanolic solution or a vehicle containing propylene glycol (PG) exceeds the penetration from an oil-in-water (O/W) cream or a petrolatum-polyethylene base (Plastibase), both without PG. The addition of PG led to higher penetration from a cream containing 0.2% HCB as compared with that from one containing 0.1% HCB. When the atmosphere on the donor side of the chamber was kept dry with a molecular sieve, the penetration from an O/W cream containing PG was enhanced, whereas that from an O/W cream without PG was substantially diminished.