The role of the 5-hydroxytryptamine (5-HT) terminals in the suprachiasmatic nucleus (SCN) in the production of the circadian variation of corticosterone secretion was investigated by lesioning the 5-HT inputs to the SCN with 5,7-dihydroxytryptamine (5,7-DHT). Vehicle-injected animals showed a normal circadian variation of corticosterone levels. In contrast, the mean corticosterone levels of the 5,7-DHT-lesioned group were intermediate between control peak and trough values, and although the individual rats showed fluctuations, no significant circadian variation was present in the group as a whole. 3H-5-HT reuptake in the SCN was reduced to 38% of the mean control level in the 5,7-DHT-lesioned group, but ventromedial hypothalamic 3H-5-HT reuptake was only decreased to 82% of control. These results indicated that the SCN 5-HT terminals may play an important role in the synchronization of the circadian variation of corticosterone secretion.