5,6-Dihydroxytryptamine is a neurotoxic analogue of serotonin which can have profound cardiovascular effects within minutes of administration in vivo (Korner and Head, 1981). These effects have been attributed to 5,6-dihydroxytryptamine-induced serotonin release, although there has been no biochemical assessment of the extent to which this occurs. The present study utilized an in vitro synaptosomal assay to determine the short-term effects of 5,6-dihydroxytryptamine on endogenous serotonin release, synthesis, storage, and metabolism. 5,6-Dihydroxytryptamine produced a rapid depletion of serotonin. At lower concentrations of 5,6-dihydroxytryptamine (0.1-1 microM), this depletion was associated primarily with an increase in the levels of 5-hydroxyindoleacetic acid, the deaminated metabolite of serotonin, with small increases in the amount of serotonin release. At higher concentrations (10-100 microM), a greater proportion of the depleted serotonin was released with less metabolism occurring. When metabolism was prevented by inhibiting monoamine oxidase, the amount of serotonin which was released equalled the amount of serotonin depletion. Thus monoamine oxidase activity was important in controlling the amount of serotonin which could be released by 5,6-dihydroxytryptamine. Further studies demonstrated that an impairment in serotonin synthesis and vesicular storage could account for the rapid depletion produced by 5,6-dihydroxytryptamine. Taken together, the results indicate that 5,6-dihydroxytryptamine acts to displace serotonin from vesicular stores into the cytoplasm where it can either be deaminated by monoamine oxidase or be released. Moreover, it is hypothesized that the intraneuronal concentration of 5,6-dihydroxytryptamine is important in determining the extent of serotonin release, because it can inhibit the deamination of serotonin by monoamine oxidase.