Abstract;Methylmercury chloride (MMC)-induced alterations in circradian sleep rhythmicity of the rat were statistically evaluated by application of cosinor method to hourly values of slow-wave sleep (SWS) and paradoxical sleep (PS) obtained4 days before and Days 23-26 after oral administration of MMC (15 mg/kg/day for 2 cosecutive days). The cosinor analysis revealed that the most prominent changefollowing MMC administration was a delay by 5 h of the circadian PS acrophase.Weattempted to elucidate the altered circadian PS rhythm in the term of brain noradrenaline (NA) metabolism. Rats administered orally with MMC at the same dose orwith a vehicle solution as a control were sacrificed by decapitation at 6 different times on Day 23 after the first administration. Noradrenaline (NA) and 3-methoxy-4-hydroxy-phenylethyleneglycol (MHPG) of the frontal cortex and the hippocampus were determined by high-performance liquid chromatography with electrochemical detection. Cortical NA levels of the MMC-administered rats were significantly lowered at 0200 h and 0600 h, as compared to those of the control rats. Cortical MHPG level of the MMC-administered rats increased significantly at 1000 h and then decreased significantly at 2200 h, whereas hippocampal MHPG levels of the MMC-administered rats decreased significantly at 1800 h and 2200 h. A plausible mechanism underlying the MMC-induced delay in the circadian PS acrophase was discussed in the light of cortical and hippocampal changes in NA metabolism.