The effects exerted by the central nervous system on endocrine activity may be compared with the influence the nervous system imposes on the body musculature. In general, the nerve supply of a muscle acts to maintain a tonic background of postural activity and also to mediate the excitation involved in reflex or voluntary movements. Similarly with many endocrine glands the nervous system maintains what may be regarded as a normal level of tonic background activity and is also instrumental in bringing about sudden changes in endocrine function according to sensory stimuli arising in the external environment. Thus the normal female rat or rabbit may show a set pattern of ovarian and thyroid function under constant environmental conditions, and this pattern may be acutely changed by the stimuli associated with, for example, sterile coitus or exposure to low environmental temperatures. The overall effects of the nervous system on gonadotrophin and thyrotrophin