Scientists at the National Toxicology Program are studying the toxicologic properties of 8-methoxypsoralen (8-MOP) with and without 320-400 nm UV (UVA). The combination of psoralen and UVA is a promising treatment for psoriasis. In this study, 8-MOP was administered to male and female Fischer 344 rats without subsequent UVA exposure for the determination of toxic effects of the psoralen alone. The drug (in corn oil) was administered by gavage 5 days/week for 90 days at doses of 0, 25, 50, 100, 200, and 400 mg/kg. The effects of toxicity were seen primarily in the 200- and 400-mg/kg dose groups, which included mortality, decreased body weight gain, and dose-related increases in liver:body ratios. On histopathology, target organ toxicity was seen in the liver, testes, and adrenals. In this study, relatively high doses of 8-MOP were tolerated in comparison to the dose of psoralen used in combination therapy clinically.