In fish, cells containing serotonin, ACh, catecholamines, NO, H(2)S, leu-5-enkephalin, met-5-enkephalin and neuropeptide Y are found in the gill filaments and lamellae. Serotonin containing neuroepithelial cells (NECs) located along the filament are most abundant and are the only group found in all fish studied to date. The presence of NECs in other locations or containing other transmitters is species specific and it is rare that any one NEC contains more than one neurochemical. The gills are innervated by both extrinsic and intrinsic nerves and they can be cholinergic, serotonergic or contain both transmitters. Some NECs are presumed to be involved in paracrine regulation of gill blood flow, while others part of the reflex pathways involved in cardiorespiratory control. There is both direct and indirect evidence to indicate that the chemosensing cells involved in these latter reflexes sit in locations where some monitor O(2) levels in water, blood or both, yet the anatomical data do not show such clear distinctions.