We review the mechanisms underlying chemoreception in fishes of the three respiratory gases, oxygen, carbon dioxide and ammonia.

We summarise the data which suggest that neuroepithelial cells of the adult gill or larval skin are tri-modal sensors of O₂, CO₂ and NH₃.

There is a specific focus on the role of the three gaseous neurotransmitters, carbon monoxide, nitric oxide and hydrogen sulphide in modulating neuroepithelial function.

It is proposed that, hydrogen sulphide stimulates while carbon monoxide and nitric oxide inhibit the ventilatory response to hypoxia in adult fish.