The effect of non-steroidal anti-inflammatory agents on u.v. radiation-induced cutaneous inflammation has been re-evaluated using a model which permits simultaneous and quantitative measurement of vasodilatation, vascular permeability and oedema formation throughout the entire time course of the inflammatory reaction. Indomethacin and phenylbutazone produced a substantial reduction in u.v. erythema during the initial stages but subsequently were far less effective, although small, significant reductions were obtained. Attempts to reverse an established inflammatory response to u.v. injury also yielded small, significant reductions in erythema but vascular permeability remained unaffected. In addition, it was found that arachidonic acid and prostaglandin E2 selectively produced vasodilatation: the vasodilator response to arachidonic acid, but not prostaglandin E2, was greatly reduced by indomethacin. It is suggested that cyclo-oxygenase derived arachidonic acid metabolites play an important role in mediating u.v. erythema only during the initial period.