Abnormal skin reactions to sunlight have been reported occasionally among patients receiving chlorpromazine* therapy.† Since this abnormal photosensitivity is a transient involvement, it appears that some factor in sunlight, present only at certain times, is responsible for triggering the eruption. In an attempt to determine what the critical light factor may be, the following study was undertaken. Method Ten patients were studied from a group of 105 receiving chlorpromazine therapy for various neuropsychiatric disorders at the Sidney Hillman Medical Center, Philadelphia.6 Five of these patients developed sunlight-induced erythematopapular eruptions with pruritus, although they had no previous histories of abnormal photosensitivity. None of these five patients had received medication other than chlorpromazine prior to the onset of the skin reactions. The other five patients had no side-effects to chlorpromazine nor had they histories of dermatitis. In addition, five volunteers who had neither received chlorpromazine nor had