['General anesthesia combined with surgery has been shown to decrease the nocturnal peak of melatonin in patients. However, the role of anesthesia itself on melatonin secretion remains unknown. We previously showed that anesthesia induced by propofol modifies the circadian time structure in both rats and humans and phase advances the circadian rest-activity rhythm in rats. In this study, we examined the secretion of melatonin during 24 h after a 30-min propofol anesthesia in rats.', 'Rats were exposed to 12-h light/12-h dark alteration conditions and anesthetized with propofol (120 mg/kg intraperitoneally) around their peak of melatonin secretion (Zeitgeber time 16). Trunk blood samples were collected at seven subsequent Zeitgeber times to assess the effects of propofol on circadian melatonin secretion.', 'Propofol modifies the peripheral melatonin by significantly decreasing its concentration ( approximately 22-28%) during the immediate 3 h after the wake up from anesthesia and then significantly increasing melatonin secretion 20 h after anesthesia ( approximately 38%). Cosinor analysis suggests that propofol induces a phase advance of the circadian secretion of peripheral melatonin.'] Does propofol anesthesia significantly alter plasma blood levels of melatonin in rats?