Myoporum laetum was collected in the municipalities of Rio Grande and Capao do Leao in winter and in Santa Vitoria in summer, autumn, winter and spring, in the state of Rio Grande do Sul, Brazil, and in the Department of Rocha, Uruguay, in winter and spring. The fresh green plant was fed to 17 sheep. All sheep developed clinical signs, except 1 that consumed only 4 g/kg bw daily during 10 d. Five of the 9 sheep dosed with 40 g/kg died. Four sheep dosed with plants from Uruguay at 40 g/kg, 6 sheep dosed with 20 g/kg, and 1 sheep dosed with 2 daily doses of 8 g/kg survived. Clinical signs were anorexia, restlessness, ruminal stasis, jaundice and dry feces with mucus or blood. All surviving sheep had photodermatitis in the face, ears, eyes and lips. Histologic lesions were characterized by periportal liver necrosis. Serum levels of AST, GGT and bilirubin were increased. M laetum from Uruguay was less toxic, suggesting a variation in toxicity among plants from different regions.