The electrochemistry of histamine and serotonin in neutral aqueous media (pH 7.2) was investigated using polycrystalline, boron-doped diamond thin-film electrodes. Cyclic voltammetry, hydrodynamic voltammetry, and flow injection analysis (FIA) with amperometric detection were used to study the oxidation reactions. Comparison experiments were carried out using polished glassy carbon (GC) electrodes. At diamond electrodes, highly reproducible and well-defined cyclic voltammograms were obtained for histamine with a peak potential at 1.40 V vs SCE. The voltammetric signal-to-background ratios obtained at diamond were 1 order of magnitude higher than those obtained for GC electrodes at and above 100 microM analyte concentrations. A linear dynamic range of 3-4 orders of magnitude and a detection limit of 1 microM were observed in the voltammetric measurements. Well-defined sweep rate-dependent voltammograms were also obtained for 5-hydroxytryptamine (5-HT). The characteristics of the voltammogram indicated lack of adsorption of its oxidation products on the surface. No fouling or deactivation of the electrode was observed within the experimental time of several hours. A detection limit of 0.5 microM (signal-to-noise ratio 13.8) for histamine was obtained by use of the FIA technique with a diamond electrode. A remarkably low detection limit (10 nM) was obtained for 5-HT on diamond by the same method. Diamond electrodes exhibited a linear dynamic range from 10 nM to 100 microM for 5-HT determination and a range of 0.5-100 microM for histamine determination. The FIA response was very reproducible from film to film, and the response variability was below 7% at the actual detection limits.