The species of the genus Lychnophora, Asteraceae, are popularly known as "arnica" and are native from Brazilian savana (Cerrado). They are widely used in Brazilian folk medicine as anti-inflammatory, to treat bruise, pain, rheumatism and for insect bites. For evaluation of acute toxicity, the ethanolic extract was given to albino female and male mice. In open-field test, the extract of Lychnophora trichocarpha (Spreng.) Spreng. (0.750 g/kg) induced a significant inhibition of the spontaneous locomotor activity and exploratory behavior of the animals were observed 1 and 4 h after administration. In traction test, the same dose reduced the muscular force 1 h after administration. The exploratory behavior reduced significantly in the group that received 0.50 g/kg, 1 and 4 h after administration of the extract. The animals that received the doses of 0.25, 0.50 and 0.75 g/kg did not show any change of blood biochemical parameters comparing to control group and showed some histopathological changes such as congestion and inflammation of kidney and liver. The dose of 1.5 g/kg caused the most serious signs of toxicity. Histopathological changes observed was hemorrhage in 62.5% and pulmonary congestion in 100% of the animals. Brain and liver congestion was found in 62.5% of the animals.