Mirtazapine (Org 3770) is a new antidepressant with prominent alpha 2-adrenergic auto- and heteroreceptor antagonistic properties and no effect on monoamine reuptake. Mirtazapine increases noradrenergic and serotonergic transmission, as measured by on-line microdialysis and by enhancement of noradrenergic locus ceruleus and serotonergic raphe nucleus cell firing. Mirtazapine has a low affinity for 5-HT1A receptors but shows 5-HT1A-agonistic-like effects in a conditioned taste aversion test and by causing lower lip retraction in rats. Mirtazapine therefore causes enhancement of 5-HT1-mediated transmission. Other studies show that both 5-HT2 and 5-HT3 receptors are specifically blocked. The enhancement of both noradrenergic and serotonergic transmission probably underlies the therapeutic activity of mirtazapine. Blockade of 5-HT2 and 5-HT3 receptors possibly prevents side effects associated with nonselective 5-HT activation and may also contribute to the anxiolytic and sleep-improving properties of mirtazapine.