Thirty-five dogs with non-seasonal atopic dermatitis were used in a double-blind, placebo-controlled crossover study of the effects of evening primrose (Oenothera biennis) oil. There was a significant treatment effect (P less than 0.05) on erythema. An analysis of the changes in plasma phospholipid levels of essential fatty acids revealed a significant (P less than 0.05) rise in linoleic acid concentration above that in the placebo group. Arachidonic acid levels in the treated group increased significantly (P less than 0.005) in the first phase and also in the second phase (P less than 0.05). In the second phase the levels of arachidonic acid in the active and placebo groups differed significantly (P less than 0.05) and there was a significant treatment effect (P less than 0.05).