Chloroform extracts of Equisetum fluviatile, E. hiemale var. affine, E. giganteum and E. myriochaetum were studied to determine diuretic activity in CD1 strain mice using hydrochlorothiazide, spironolactone and furosemide as standard drugs for comparison. It was found that the most active plant was E. hiemale var. affine, followed by E. fluviatile, E. giganteum and E. myriochaetum, producing an effect similar to that of hydrochlorothiazide in relation to the excretion of sodium, potassium and chloride.