To date about thirty peptides--low-molecular-weight, single-chain amino acid compounds--are known to be distributed widely in the central nervous system within selective neuron pathways. These findings, combined with a large body of neuropharmacological, behavioral, and electrophysiological data, open new horizons in neurobiology, force a reexamination of old and accepted hypotheses, and hold important implications for the clinician. There is evidence that substance P and the opioid peptides play a major role in the pain pathway, particularly at the level of the spinal cord. Available evidence also implicates vasoactive intestinal polypeptide in the control of cerebral circulation, cholecystokinin in the regulation of appetite, and vasopressin and adrenocorticotropic hormone in memory. Many questions, however, remain. For most peptides there is little information on mechanisms of biosynthesis, release, interaction with receptors, and termination of biological effect. Another important question is the interaction of peptides with other neurotransmitters. The evidence that both "classic" neurotransmitters and peptides can be found in the same neuronal necessitates reformulation of Dale's "one neuron, one neurotransmitter" hypothesis. It may be that a single cell, while containing different classes of neurotransmitter, will contain only one member of any particular class. It is not too early to speculate on the role of the numerous and diverse peptides in neuronal tissue and on the implications of peptide abnormalities in a variety of neurological diseases. The answers to these and other questions pose a fascinating challenge to neurobiologist and clinician alike.