The present research was conducted to test the usefulness of a new index, defined in a previous work, to determine the diuretic power (DP) of medicinal plants and drugs. Plant materials, extract preparation, animals, drugs and procedures for obtaining samples, and measurement units used have been extensively described in previous works. Urine was collected in a graduated cylinder during the 8 h period after the administration of plants and drugs. Na⁺ and K⁺ concentrations were measured by flame photometry, and Cl^- concentration by direct potentiometry using a selective chloride electrode and a reference electrode. Specific electrical conductivity of thermostated fresh urine was determined with a conventional conductivity meter. The results obtained after oral administration of the methanolic extracts of three plants (Artemisia thuscula (AT), Withania aristata (WA) and Smilax canariensis (SC)) and the Hydrochlorothiazide (HCTZ) used as reference drug can clearly show that the HCTZ produced a 40% increase in DP, with respect to the control group, and the studied plants show an increase up to 21%, in a dose-dependent manner in all the cases. Finally, when DP values were compared with other common indexes, diuretic index (DI) and saluretic index (IS), the sequence variation of the three indexes was the same for HCTZ, WA and SC. In the case of AT, small discrepancies were observed that may be due to experimental errors. Briefly, the new and easy method we propose for estimating the diuretic activity of drugs or plants is able to determine the diuretic effect of a particular drug or plant with respect to a specific control value. Its application requires only two experimental parameters: the volume of urinary excretion and the specific electrical conductivity of the sample.