Abstract: Ruta graveolens Linn (Family: Rutaceae) leaves are used in Sri Lankan traditional medicine as a diuretic but this effect is not scientifically validated. This study has evaluated the diuretic potential of R. graveolens leaves in rats using a Hot Water Infusion (HWI). Different concentrations of HWI (4.5, 6.75, 9.0 mg mL-1) or vehicle or furosemide (13 mg kg-1) were orally administrated (n = 6 per group) to hydrated rats and their urine output was monitored hourly for 6 h. Urinary pH, specific conductivity, specific gravity, Na+, K+, Cl¯ levels and creatinine clearance were determined (with the highest dose and control). Using these data standard urinary indices were calculated. Further, subchronic toxicity was examined in terms of serum Glutamic Oxaloacetic Transaminase (GOT), Glutamic Pyruvic Transaminase (GPT), urea and creatinine levels and overt signs. HWI increased the urine output markedly in a dose-dependent manner. The high dose of HWI was almost equipotent to furosemide (in terms of diuretic activity). The onset of diuresis was very rapid (within 1 h) and lasted throughout the studied period. HWI also caused significant increase in specific gravity, specific conductivity, creatinine clearance, Na+ and K+ levels, thiazide secretion index, urine alkaline index, diuretic action, Na+ and K+ saliuretic indices and significant decrease in carbonic anhydrase index. Further, no evidence of subchronic toxicity was seen. R.graveleons leaves exhibits safe and strong oral diuretic activity as claimed in Sri Lanka traditional medicine. This action is mediated via multiple mechanisms: thiazide like activity, inhibition of carbonic anhydrase activity and increase in glomerular filtration rate.