A few chemicals serve both as hormones in the endocrine system and as chemical signals in the nervous system. Epinephrine, for example, functions in the vertebrate body as the so called "fight–or–flight hormone" (produced by the adrenal medulla, an endocrine gland) and as a neurotransmitter, a local chemical signal that conveys messages between neurons in the nervous system.

In addition, the nervous system plays a role in certain sustained responses for example, controlling day/night cycles and reproductive cycles in many animals often by increasing or decreasing secretion from endocrine glands.

Thus, although the endocrine and nervous systems are anatomically distinct, they interact functionally in regulating a number of physiological processes.