The demonstration of mammalian hormone and neu-rotransmitter molecules in the simplest organisms suggests that the nervous and endocrine systems share a common biochemical ancestry (1, 2). Although both systems subserve the functions of communication and maintenance of homeostasis within the organism, they have diverged significantly during evolution. Specificity of action within the nervous system is conferred through welldefined anatomic connections, whereas in the endocrine system the unique interaction between a secretory and target cell depends on the specificity of hormone binding to its receptor. The short diffusion distance within the synapse permits the resolution of discrete events on a millisecond time scale. Information may therefore be transmitted rapidly within the nervous system through modulation of the frequency of synaptic activity as well as through activation of circuits containing specific neurotransmitter molecules. The separation of secretory and target cells prevents rapid transmission of information within the endocrine system. There is, however, no theoretical obstacle to frequency encoding of endocrine information provided that the secretory interval exceeds the time required for circulation and diffusion of a hormone molecule into the intercellular space. Indeed, the close similarities between many other aspects of nervous and endocrine function make it reasonable to postulate a physiological role for episodic hormone secretion. Although a circadian periodicity of many endocrine rhythms has been appreciated for years (3), the first evidence for frequency encoding of endocrine information came from studies of the ultradian secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH)(4-6).