A UV spectrophotometric method for the determination of hydralazine hydrochloride in tablets was collaboratively studied by 5 laboratories. The method is based on conversion of hydralazine to a tetrazolo [5,1-alpha]phthalazine derivative which shows an absorption maximum at about 274 nm. Each collaborator received blind duplicate samples of 2 commercial powdered composites from 10 and 100 mg tablets, and 1 synthetic tablet formulation. Each collaborator also received a set of 10 tablets for determination of content uniformity. The pooled mean recovery of hydralazine hydrochloride from the synthetic formulation was 101.2 +/- 0.94%. The mean assay values for 10 and 100 mg tablets were 95.6 +/- 0.98 and 101.0 +/- 0.73% of the declared amounts, respectively, with corresponding CV values of 1.02 and 0.73%. The pooled mean for individual tablet assay was 99.8 +/- 3.26% of the declared value, with a CV of 3.29%. The method has been adopted official first action.