Herbal remedies are widely used in Moroccan pharmacopoeia. We assessed the diuretic effect of two medicinal plants: Rosmarinus officinalis L., Labiatae, and Centaurium erythraea L., Gentianaceae, both reputed for the treatment of urinary ailments. To determine the action of these herbs on urinary volume (UV) and the excretion of sodium (U(Na)V), potassium (U(K)V), and chloride (U(Cl)V), the aqueous extracts of both plants were administered daily to Wistar rats for 1 week. The concentration of electrolytes and urea in plasma and creatinine clearance were also investigated. Daily oral administration of the aqueous extracts of R. officinalis and C. erythraea at the dose of 10 ml/kg of 8 or 16% extract in distilled water significantly enhanced diuresis in rats compared to the control group from the fifth day of treatment. For R. officinalis at the dose of 8% the peak of urinary excretion of sodium, potassium and chloride was reached after 6 days of treatment (P<0.001). The aqueous extract of of R. officinalis at the dose of 16% did not significantly affect the excretion of water and electrolytes over a similar period but slight increases in urinary excretion of sodium and chloride on the seventh day and of potassium on the sixth day (P<0.05) were observed. No increase was recorded for 24 h urinary excretion of Na+, K+ and Cl- during the first 4 days of treatment for the groups treated with C. erythraea at the doses of 8 and 16% whereas their effects on the same parameters were highly significant thereafter. No change was observed in plasma electrolytes and urea in any group, except for a decrease in sodium and chloride concentration in the group treated with 16% of R. officinalis. A decrease in creatinine clearance was demonstrated after treatment with 8% of R. officinalis and C. erythraea. Our findings demonstrate a diuretic effect of aqueous extracts of R. officinalis L. and C. erythraea L. with the most effective dose for water and electrolyte excretion being 8% for both plants.