Circadian rhythms in blood pressure and norepinephrine in genetically hypertensive and normotensive rats

1. 1. Circadian changes in systolic blood pressure (B.P.) were determined in genetically hypertensive (GH) and normotensive rats and correlated with changes in norepinephrine (NE) levels in various brain regions, the heart and the adrenal glands. 2. 2. In the hypothalamus of GH rats a relatively low level of NE was found when the B.P. reached a peak level during the late dark phase, and a relatively high level of this catecholamine was found when the B.P. was low in the light period. 3. 3. The most significant difference between hypertensive and control rats in NE concentration in the brainstem and heart was found in the early light hours, when the B.P. was relatively low.