Octopamine is a normally occurring amine in nervous tissues in many species of animals. In mammals, octopamine is formed from tyramine through beta-hydroxylation by DBH in the sympathetic nerves, and it is partially stored in nerve endings with a subcellular distribution similar to that of NE. It is capable of replacing NE in its storage sites, and it is released by sympathetic nerve stimulation. High concentrations are found in the crustacean central nerve cord. Specific octopamine-containing cells have been identified in Aplysia and other lower animals. On the basis of neurophysiological evidence, the existence of specific octopamine receptors in Aplysia has been postulated. In insects, octopamine produces specific biochemical responses such as increased synthesis of cyclic AMP and phosphorylase activation. The present evidence strongly suggests that octopamine may function as a neurotransmitter in lower animals. Although the physiological role of this amine has not been established in mammals, it appears likely that octopamine may function as a cotransmitter together with NE in the peripheral sympathetic nervous system.