The effects of bath-oil and tap-water baths were studied by non-invasive bioengineering methods. Measurements of water evaporation, electrical conductance and capacitance demonstrated an increase in cutaneous hydration for 20 min after both types of bath, with the larger increase occurring within the first 10 min. A small but significantly greater amount of water (12-27%) was bound in the skin following use of bath oil. However, measurements of evaporation, conductance and capacitance indicated no clear difference in the skin surface hydration following bath-oil and tap-water baths. Thus, the increase in the water-holding capacity of the skin resulting from bath oil is slight and of no real importance for skin-surface hydration immediately after bathing. There was no difference between 5-min and 20-min baths. Oil baths resulted in an increase in skin-surface lipids lasting at least 3 h, comparable to the effect of traditional moisturizing lotions. This lipidization of the skin surface may have protracted effects. In conclusion, the value of bath oil lies mainly in the general lipidization of the skin with potential improvement in dryness and scaling, i.e. effects which are complex and protracted. The direct hydration of the skin is of short duration and comparable to a tap-water bath. In comparison with lotions, use of an oil bath has the disadvantage that it is not practical for repeated daily treatment over the long period which is necessary for therapy to be effective. The present study on normal skin does not take into account other effects of bathing with or without the addition of oil.