We have investigated the correlation between urinary isatin concentration and MAO-B inhibitory activity. A sensitive enzyme-linked immunosorbent assay (ELISA) for the measurement of isatin in urine was developed. There was a strong correlation between isatin concentrations determined by this method and the established GC/MS technique. In two sets of urine samples from healthy individuals ( N = 63 and N = 21). we have shown that there is no significant correlation between isatin concentration and MAO-B inhibitory activity ( r = -0.1013 and -0.1597, respectively). Based upon I C , , measures, the concentration of isatin in normal urine ( N = 10) was 3-5 times less than that which could account for the detected MAO-B inhibitory activity. Furthermore, Lineweaver-Burk plots from urine samples ( N = I?) compared with isatin showed that the inhibition by isatin was competitive whereas urine samples could elicit either mixed or non-competitive inhibition. We conclude that the MAO-B inhibitory activity in urine cannot be attrihuted to isatin alone and another inhibitor(s) appears to be present. KEY wom---isatin; trtbulin: endogenous MA0 inhibitor(s); MAO-B inhibitor; Lineweaver-Burk plot; ELISA; human urine Endogenous monoamine oxidase (MAO) inhibitory activity was first demonstrated in human urine in 1980' and was later named tribulin.' Urinary tribulin levels in man appear to be raised in various psychophysiological conditions associated with stress, arousal or anxiety.' ' Increases in rat brain tribulin levels have been observed following ~t r e s s . ~. ~ In 1988, using direct probe mass spectrometry. isatin was identified as the major component of urinary tribulin."' It? vitro. isatin has been shown to elicit a potent inhibitory effect on M A 0 type-B activity (ICs0 = 3 pM) with lower potency on MAO-A (ICso = 30 p M ) and at central benzodiazepine receptors (ICj,, = 113 pM).".