To understand the biological function of taurine, a study of taurine kinetics in the cat was undertaken. This paper describes a method developed for the accurate determination of 15N-taurine enrichment in cat urine by gas chromatography-mass spectrometry. 15N-Taurine was given to six animals as an oral bolus dose of 20 mg/kg body weight, and the urine was pooled on a daily basis. The hydrolysed or non-hydrolysed urine samples (for total and free taurine, respectively) were directly derivatized without further purification. The N-pentafluorobenzoyl di-n-butyl amide derivative obtained was analysed, and the fragment [M-(di-n-butyl amide)]+, carrier of the labelled nitrogen atom, was selectively recorded at m/z 302 (14N-taurine) and m/z 303 (15N-taurine). Calibration curves prepared in hydrolysed and non-hydrolysed urine samples spiked with 15N-taurine gave similar slopes to the calibration curve prepared in water. The average coefficient of variation observed for the mole percent excess in the non-hydrolysed samples was 1.22% (n = 92) and for the hydrolysed urine 1.00% (n = 98). There was no significant difference between free and total taurine enrichment. The half-life of taurine in cat body was found to be 29.3 +/- 2.9 h and 35.0 +/- 1.4 h for free and total taurine, respectively (non-significant). The taurine body pool, calculated by extrapolation of the curve to zero time, had a value of 137 +/- 22 ng/kg and 157 +/- 11 mg/kg for free and total taurine, respectively.