Microdialysis probes inserted into chronically implanted guide shafts allowed the collection of serotonin and 5-hydroxyindoleacetic acid (5-HIAA) from the lateral hypothalamus of rats during feeding behavior. After the collection of baseline samples, animals were offered a palatable diet that they could only see and smell for 60 min, then they were allowed access to the food for an hour. An additional three samples were collected after food was removed. Extracellular serotonin increased during the first half hour of access when the animals actually ate the food, and then returned to baseline level throughout the remainder of the test. 5-HIAA decreased gradually with no increase during feeding. These data suggest that eating a meal of palatable food causes a short-term increase in extracellular serotonin in the lateral hypothalamus. This increased serotonin may play a role in the control of lateral hypothalamic feeding and reward.