The circadian levels of norepinephrine and serotonin in the caudate nucleus, cerebellum, cortex and midbrain were simultaneously determined fluoro‐metrically every three’ hours in control rats and in rats pretreated intraperi‐toneally with sublethal doses of phenobarbital sodium. All animals were adapted for minimum period of three weeks in an environmental room equipped with an automatically‐timed photoperiod lasting from 08:00 to 20:00 h daily. The results‐of this experiment show that the levels of norepinephrine and serotonin in all the brain areas studied vary diurnally. Phenobarbital sodium pretreatment alters the levels and/or phase‐shifts the circadian pattern of these biogenic amines.