In this study, we investigated the effect of p-menthane monoterpene enhancers on the penetration of propranolol HCl across newborn pig skin. These permeation experiments showed that hydrocarbons had mild enhancing activity when compared with terpenes with polar functional groups, such as alcohol or ketones, but that the enhancing activity was stronger than that found with terpenes with added lipophilic groups, such as trifluoroacetate. The greatest penetration enhancing activity was found when the aromatic terpenes, carvacrol and thymol, were used as enhancers. The penetration enhancing activity of hydrocarbon terpenes was not affected by either the number or the position of the double bonds. In addition, there was no significant difference between the racemate and the pure enantiomers of menthol and carvone in enhancing permeation of propranolol HCl. Retention of the model drug in the skin was also promoted by the presence of terpenes. There was no clear correlation between permeability values and skin retention of the model drug. Partitioning of propranolol HCl into the stratum corneum was increased following treatment with terpenes in 40% ethanol. The increased partitioning of the model drug was possibly due to the effect of the terpenes dissolved in ethanol rather than a direct effect of terpenes themselves. The degree of skin partitioning of propranolol HCl correlated well with the permeability values but appeared to be unrelated to the amount of skin retention.