[15N]Taurocholate has been prepared and used as a simple urine test substrate as a substitute for14C- and13C breath test substrates. [15N]Taurocholate was administered instead of [15N] glycocholate assuming that taurine is mainly used in the cholate cycle. Differences were found between healthy and diseased subjects, although deconjugated [15N]taurine was recovered in urine much less than expected. To get additional information on the actual excess of free taurine, [15N]taurine was administered orally in parallel investigations. Apparently, large fluctuations in the whole body taurine metabolism are responsible for the unexpected results.