The concentration of N-butyl-N-(3-carboxypropyl)nitrosamine (BCPN), which is the major metabolite of the carcinogen N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN), was measured in the urine, thymus, liver, kidney, and bladder of rats orally administered with BBN. Since BCPN is a carboxylic acid, it forms an ester with 9-anthryldiazomethane (ADAM), which is a fluorescent labeling agent highly sensitive to carboxylic acids. Thus, BCPN and ADAM were reacted at 40 degrees for 1 hr, and the resulting ester was separated and measured by high-performance liquid chromatography (HPLC) with a reverse-phase type column. The range of measurement was 0 to 40 micrograms/ml, and the coefficient of variation (CV) was 3.8%. When 0.025% BBN was given orally to rats in tap water, the BCPN concentration in the urine was very high at 220 micrograms/ml, while it was 0.15 microgram/100 mg in the wet tissues of the thymus, 0.35 microgram/100 mg in the liver, 0.40 microgram/100 mg in the kidney, and 1.2 microgram/100 mg in the bladder. The BCPN concentration in the bladder, in which tumors are induced by the administration of BBN, was thus higher than those in the other organs.