The effect of irradiation with UV light on herpes simplex virus infection in the mouse ear was investigated. Irradiation 2 days before infection had no effect on the titre of virus in the skin 3 days after infection. In ears irradiated 24 hours or immediately before infection virus titres measured 3 days after infection were depressed. Irradiation 2 days after infection enhanced virus titres. The dose of irradiation used did not directly inactivate herpes simplex virus in the ear. The results are discussed with reference to theories of herpes simplex reactivation.