In the Sri Lankan traditional medicine, Spilanthes acmella Murr. (Family: Compositae) flowers are claimed to possess powerful diuretic activity. However, as yet, the diuretic potential of these flowers is not investigated by scientifically controlled studies. The aim of this study was to evaluate the diuretic potential of Spilanthes acmellaflowers in rats using a cold-water extract (CWE). Different concentrations of CWE (500, 1000, 1500 mg/kg) or vehicle or furosemide (13 mg/kg) were orally administered (N = 6 per each treatment group) to hydrated rats and their urine output was monitored at several intervals of time (1-5 h). The highest dose of CWE significantly (P < 0.05) and markedly increased the urine output. The onset of this diuretic action was extremely prompt (within 1 h) and lasted throughout the studied period (up to 5 h). The peak effect was evident between 1 and 2 h. Further, the intensity of diuresis induced by the CWE in the first hour was almost similar to that of furosemide. Spilanthes acmella CWE also caused marked increase in urinary Na(+) and K(+) levels and a reduction in the osmolarity of urine suggesting that it is mainly acting as a loop diuretic. It may also inhibit ADH release and/or action. It is concluded that the Spilanthes acmella CWE has strong diuretic action as is claimed.