1. Exposure of the skin of rats to u.v. light (greater than 295 nm) for 30 s or longer elicited a delayed erythema response, the rate of onset increasing with period of irradiation. The erythema was still present at 24 h and was replaced by scab formation in 48 hours. 2. Both topically applied steroidal and non-steroidal anti-inflammatory drugs reduced the erythema formation when administered immediately after u.v. exposure. Propyl gallate, an antioxidant with sun screening properties in man, also possessed topical anti-erythemic activity. 3. Both steroidal and non-steroidal anti-inflammatory drugs, systemically administered 1 h before u.v. exposure, reduced and erythema. However, the steroidal compounds were less effective than the non-steroids and reduced the intensity of erythema by less than 50%. Antagonists of 5-hydroxytryptamine (5-HT) reduced the erythema but several other drugs with different pharmacological activities were ineffective. 4. Neither topical nor systemic treatments of any of the drugs examined suppressed the scab formation at 48 hours. 5. These results and those using other selective blocking agents indicate that in the mediation of the erythema reaction prostaglandins may play a major role and 5-HT perhaps a minor one but that H1 histamine receptors and alpha- and beta-adrenoceptors have no significant role.