The use of brucine for the colorimetric determination of the oxidisable oxides of nitrogen in cigarette smoke is described. The oxides of nitrogen in the cigarette smoke are converted to nitrate with dilute aqueous hydrogen peroxide, and the nitrate is determined by using a modification of the method of Robinson, Allen and Gacoka. Results are presented for the determination of the oxidisable nitrogen oxides present in the smoke of various types of cigarettes. The coefficient of variation of the method is 7·7 per cent.