Permeation of zidovudine (3'-azido-3'-deoxythymidine, AZT) and probenecid from oily bases containing an alcohol through rat skin was examined. Isopropyl myristate (IPM), as an oily vehicle, showed a penetration enhancing effect for AZT and probenecid. Ethanol, n-propanol, and n-butanol were used as additives in IPM and were examined for their own permeation and the enhancing effect on the permeation of AZT and probenecid. ir;he skin permeation of AZT and probenecid from IPM was enhanced by addition of the alcohol in IPM. The degree of the enhancement was decreased with increasing lipophilicity of the alcohol used. The permeation rate of the drug from those systems was shown to be governed by penetration-enhancing effects of the oily base and alcohol, and the penetration of the alcohol itself through the skin.