Optimized operating conditions previously developed for the determination of neuroactive indoleamines and metabolites were adapted to meet the requirements of thermospray liquid chromatography-mass spectrometry (LC-MS) in terms of the ammonium acetate buffer system needed in this technique. Mass spectra were obtained for nineteen indolic compounds in both the positive and negative ion modes. The positive thermospray mass spectra of indoles with a free primary amino group are characterized by the base peak at [M + H]+, whereas the alcohol and acid metabolites show the base peak at [M + NH4]+. In the negative mode only amino acids and acids give good mass spectra with base peaks at [M - H + ACOOH]-. Detection limits by selected ion monitoring were of the order of 50-100 pg SIM on-column, allowing the direct determination of endogenous serotonin in an extract from rat hypothalamus. Quantitation was performed by isotope dilution MS. In the same way 5-hydroxyindoleacetic, indoleacetic, indolepropionic and indolelactic acids in urine were directly determined in an ethyl acetate extract from acidified urine samples. Likewise, gamma-aminobutyric acid and tricyclic antidepressants gave detection limits of 10 pg whereas only nanogram sensitivity could be achieved with catecholamines.