The "chá de bugre", Casearia sylvestris Swartz has been popularly used for several purposes. More recently, abortive characteristics have been attributed to it. This possibility has prompted us to evaluate the toxicity of its crude natural products and their "in vitro" effects on uterine motility. The median lethal dose of the aqueous extract from C.sylvestris leaves is 1792g/kg, in albino mice. With the aqueous solution of the ethanol extract we have observed "in vitro" on rat uterine spontaneous motility; increase of frequency of contraction, elevation of resting tone and decrease in the amplitude of contraction. This solution of ethanol extract has reduced the maximum response and increased the median effective dose (ED50) to oxytocin. The aqueous mecerated solution, a kind of galenical preparation, has increased all the parameters observed on uterine spontaneous motility; it has also affected the cumulative dose-response of oxytocin by increasing the maximum response and decreasing the ED50. These results suggest that extracts from leaves of C. sylvestris are able to modify, "in vitro", uterine activity. These data are in agrument with the popular use of this plant as abortive, in our country.