The question of whether serotonin is deaminated by MAO before it can be released or after release has occurred was investigated by studying the 5-HT behavioral syndrome in acutely reserpinized rats. The release of serotonin from vesicles by reserpine does not produce the serotonin behavioral syndrome which is an in vivo index of serotonin release and receptor activation. However, if rats are first pretreated with a nonselective monoamine oxidase inhibitor (e.g., tranylcypromine), the injection of reserpine is followed by symptoms which are characteristic of the behavioral syndrome including forepaw treading, hindlimb abduction and head weaving. Neither selective MAO-A or -B inhibition with clorgyline or deprenyl, respectively, nor inhibition of serotonin reuptake with fluoxetine prior to reserpine produced the serotonin behavioral syndrome. However, the combination of clorgyline and deprenyl followed by reserpine does so. These behavioral data along with neurochemical analyses of serotonin and 5-hydroxyindoleacetic acid levels lead to the conclusion that serotonin does not have to be released before it is metabolized to 5-hydroxyindoleacetic acid. Consequently, the levels of 5-hydroxyindoleacetic acid in brain reflect MAO activity and not serotonin release or utilization.