Isatin has recently been identified in rat tissues and normal human urine, where it forms the major proportion of the endogenous monoamine oxidase inhibitor, tribulin. In this paper, we show that isatin, measured by gas chromatography/mass spectrometry, has a distinct regional distribution in rat tissues, with highest concentrations in seminal vesicles (1.6 ?g/g) and vas deferens (3.4 ?g/g). There was also a discontinuous distribution within rat brain, concentrations being highest in the hippocampus (0.13 ?g/g).