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The role of serotonin in controlling feeding in flesh flies is examined. Amount of feeding was recorded over 6 h for flies injected with serotonin or saline. The proportion of time spent on various behaviors over a period of 1 h was recorded after the injection of serotonin or saline or no injection. Corresponding electrophysiological measurements were made on serotonin and saline-injected flies. The release of serotonin as a consequence of feeding was also examined. The subesophageal ganglia of flies taken before or after 2 days of sugar feeding were examined immunocytochemically. Serotonin injection decreased feeding in flies compared to saline-injected flies. All behaviors measured decreased after serotonin injection, except for resting, which increased, and grooming, which decreased in both serotonin- and saline-injected flies. A marked reduction in electrophysiological responses to sucrose was seen in serotonin-injected flies. Specific cells of the subesophageal ganglion showed significantly less serotonin immunoreactivity in fed flies compared to flies that had not yet fed. The role of serotonin in affecting the physiology of feeding in insects is discussed.