This paper discusses the current evidence supporting the notion that endogenous carbon monoxide (CO) is a modulator of neuroendocrine function. CO is normally formed in the body during the enzymatic catabolism of heme moieties by heme oxygenase (HO). Three HO isoforms have been described to date: HO-1, HO-2 and HO-3. In the brain, CO is principally generated by HO-2 but, in discrete brain areas such as the paraventricular nuclei of the hypothalamus, a role for HO-1 is also possible. Moreover,...