Transepidermal water loss (TEWL) was measured as assessment of barrier function in house dust mite-sensitized atopic (n = 24) and normal (n = 21) beagle dogs before and after allergen challenge. Dogs of the two groups were matched for age and housed in the same environment. Ten sites were evaluated including both areas of high and low predisposition to atopic lesion formation. On day 0 (before allergen exposure) and day 3 (3 days after allergen exposure), TEWL measurements were taken using an open-chamber evaporimeter. On day 0, significant differences (one-way ANOVA) were found between normal and atopic dogs in the chin, pinna, periocular region, axilla, antebrachium and thorax. Within the atopic group, a significant increase (one-way ANOVA) in TEWL was found in the axilla on day 3. Such a difference was particularly marked in young dogs (chin, pinna, axilla, antebrachium and groin). Within the normal group, allergen challenge did not change TEWL except for the chin. When atopic and nonatopic sites were compared within the atopic group, a significant difference in TEWL was found on day 0. A two-way ANOVA showed a significant effect of site, a significant effect of time but no site x time (i.e. allergen exposure) interaction. It is concluded that the house dust mite-sensitized atopic beagle dogs used in this study have altered barrier function in 'atopic' areas and that such differences are more evident in young individuals and are aggravated by allergen exposure.