The effects of Clematis montevidensis Spreng. (Ranunculaceae) on urinary excretion of water, sodium and potassium were investigated in rats loaded with isotonic saline solution. The data reported in the present work indicate that the infusions of the root and aerial part of Clematis montevidensis showed a moderate diuretic activity. This effect could be due, at least in part, to the presence of oleanolic acid isolated from this plant.