Abstract: Therapeutic activity, and in particular the ability to induce diuresis or saluresis, has been discovered in 4-hydroxy- or 4-mercapto-2-aryl-1,2-dihydrophthalazine-1-acetic acids, the tautomers of these namely 4-oxo or thio-2-aryl-1,2-tetrahydrophthalazine-1-acetic acids, and in the ester and alcohol derivatives of the acids, and in the acyl derivatives of the 4-hydroxy or mercapto compounds and in the 3-alkyl derivatives of the 4-oxo or thio compounds. Therapeutic compositions and methods using the compounds are described, as also are novel compounds.