Methenamine (hexamethylenetetramine) and its hydrolysis product formaldehyde are determined in the presence of each other in urine by a spectrophotometric method. Formaldehyde is assayed by a colour reaction with tryptophan, sulphuric acid and ferric chloride after precipitating methenamine by three treatments with mercuric chloride. Methenamine is indirectly analysed by hydrolysis to formaldehyde with dilute hydrochloric acid. Formaldehyde levels as low as 5.0 micrograms ml-1 in the presence of methenamine concentrations as high as 2.5 mg ml-1 can be assayed. Of practical significance is the feature that urine may be stored up to 1 weeks for analysis, by appropriate dilution and freezing, without excessive loss of methenamine or formaldehyde. The method was applied to the dtermination of the bioavailability of methenamine hippurate in ten human volunteers.