Abstract: Certain derivatives of oxophthalazinyl acetic acids of the formula ##STR1## wherein R.sub.1 is hydroxy or a prodrug group, and R.sub.2 and R.sub.3 are independently hydrogen, fluoro, chloro or trifluoromethyl, except that R.sub.2 and R.sub.3 are not both hydrogen, or a pharmaceutically acceptable base addition salt of a compound of formula I wherein R.sub.1 is hydroxy, are capable of lowering blood uric acid levels in mammals. These oxophthalazinyl acetic acid derivatives are for instance useful for treating gout and gouty arthritis in a mammal .