Species of the Lychnophora genus are plants native to Brazil, popularly known as "Brazilian arnica" and used in folk medicine as alcoholic and hydro-alcoholic preparations for the treatment of bruises, inflammation, pain, rheumatism and insect bites. The present study aimed to evaluate the safety of the use of Lychnophora pinaster Mart., Asteraceae. Acute toxicity of the crude ethanolic extract was evaluated by administration of the extract by oral route to male and female Swiss mice. A single extract dose of 125, 250 or 500 mg/kg was administered and the effects on spontaneous locomotor activity, exploratory behavior, muscle strength, body weight, food and water consumption, relative organ weight, histology, as well as hematological and biochemical parameters were evaluated. The three doses administered to the animals did not cause muscle tone alterations, but doses of 250 and 500 mg/kg induced a significant inhibition of the spontaneous locomotor activity and exploratory behavior of the animals in open-field test. There was no alteration to hematological parameters and consumption of water and food, body weight variation and organs relative weight. Changes were observed in AST and ALT during assessment of biochemical parameters. The histopathological evaluation showed that the extract provoked cellular alterations, such as vacuolar degeneration and inflammation in kidneys and liver at all doses. Liver morphometric analyses of male and female mice showed that the extract did not have dose-dependent effects. Although females showed a significant increase in inflammatory cells, the effect was not dose-dependent.