IN THE last decade, the explosive increase in our knowledge of neuropeptides has demonstrated that the release of peptides from the gastrointestinal tract can modulate behavior; and, contrariwise, gastrointestinal peptides have been shown to be synthesized and released in the central nervous system and act as neurotransmitters. This review will concentrate on the role of peptides as modulators of feeding behavior and weight regulation. The complexities of the neurochemical regulation of food intake are not surprising when one considers the varied inputs involved in the determination of feeding behavior. With depletion of an organism's energy stores, a series of signals arise which force the animal to hunt for food. When the food source is found, the animal needs to do a cost-benefit analysis, i.e. a determination of whether the potential danger and the energy expenditure involved are justified in terms of the calories to be gained and secondarily, to decide whether or not the food is suitable for consumption.