A rapid and sensitive method for the determination of mercury in urine is described. It is based upon wet digestion of 1-ml samples of urine by potassium permanganate-sulphuric acid at room temperature overnight, reduction of mercury(II) to metallic mercury by tin(II), evaporation of mercury vapour from the solution by an air current at room temperature and determination of mercury by cold-vapour atomic-absorption measurement at 253·7 nm. Mercury determination on digested samples can be carried out at the speed of about 1 sample minute–1. The amounts of chemicals and of glassware needed are much less than those required for the dithizone method. Detection limit is 2 ng of mercury in 1 ml of urine. The proposed method has been compared with dithizone analysis in a series of urine samples with fairly good results. Chemical interference from various substances is discussed.