The basic level of methionine-enkephalin (MEK) in rat medulla oblongata plus Pons was thehighest at 05: 00 o'clock and the lowest at 23: 00. In both the hypothalamus and the striatum at 05: 00 MEK levles were respectively higher.than those at 11: 00

17: 00 amd 23: 00. In both thehippocarnpus and the midbrain at 05: 00 and 23: 00 they were respectively higher than those at 11:00 and 17: 00. In the cortex it was higher at 05: 00 than those at 11: 00 and 17: 00Electroacupuncture (EA) at 05: 00 increased the MEK content in the tnedulla oblongata pluspons and decreased the MEK content in the hypothalamus. EA at 11: 00 produced decreases of MEKconcentrations in the medulla oblongata plus pons and the midbrain and increases of MEK content inthe cortex. At 17: 00 it increased the MEK level in the hypothalamus and at 23: 00 it reduced thelevel of MEK in the hippocampus.The results suggest that there are very marked circadian rhythrns in the MEK basic levels of thesix discrete hrain regions observed and EA at different hours produces different effects on brain MEKlevels.