Biological rhythms are thought to be related to the pathogenesis and therapy of various diseases including depression. Here we investigated the influence of circadian rhythms on the antidepressant activity of the dual-action serotonin-noradrenaline reuptake inhibitor (SNRI) milnacipran. Rats administered milnacipran in the morning (8:00 a.m.; zeitgeber time [ZT]1) or in the evening (8:00 p.m.; ZT13) were analyzed in a forced swim test (FST). At ZT1, the rats' immobility was reduced and the swimming was increased, whereas at ZT13, their climbing was increased. These results suggest that the serotonergic and noradrenergic systems are preferentially affected at ZT1 and ZT13, respectively by milnacipran. We analyzed the plasma and brain levels of milnacipran after administration, and there were no differences between ZT1 and ZT13. The circadian rhythm of monoamine neurotransmitters was analyzed in several brain regions. The serotonin turnover showed rhythms with a peak during ZT18-ZT22 in hippocampus. The noradrenaline turnover showed rhythms with a peak during ZT22-ZT2. There was a difference of approx. 4 h between the serotonergic and noradrenergic systems. This time difference might be one of the factors that affect the action of milnacipran and contribute to the dosing time-dependent behavioral pattern in the FST.Key words circadian rhythm; antidepressant; forced swim test; milnacipran; serotonin; noradrenaline Many physiological functions have a rhythm with a period of 24 h. In mammals, circadian clock systems govern almost all organs, and they affect the pathophysiology of various diseases such as cancer, cardiovascular diseases, and psychiatric diseases.1-4) The circadian clock systems also affect the efficacy of medication by influencing the pharmacokinetics and/or pharmacodynamics of the drug.5-7) The efficacy of various drugs varies according to the dosing time.8) It is important to consider the chronopharmacological profiles of each drug in order to provide effective medication.Depression is one of the diseases thought to be affected by the circadian system. Depressive symptoms relate to the chronotype, 9) the symptoms of the depression show diurnal variations, 10) certain types of depression are developed according to the length of daylight, 11) and the modification of the circadian systems is sometimes effective for treating patients.12-14) The monoamine hypothesis proposes that the reduction of monoamine systems in the brain causes depression. However, the interactions between the circadian system and various pathophysiologies are not yet thoroughly understood.Many therapeutic drugs have been developed for depression, and various drugs such as tricyclic antidepressants, selective serotonin reuptake inhibitors (SSRIs), and serotoninnoradrenaline reuptake inhibitors (SNRIs) are currently used in clinical settings. Most of these drugs modify the monoaminergic systems in the brain to exert their antidepressant activity.In rodent models used to assess the effectiveness of antidepressants, the forced swim te...