Temporal changes in the metabolism of norepinephrine (NE) were found in female rats following the administration of estrogen (15 micrograms biweekly) for 3 weeks, in regions of the brain believed to be involved in the control of blood pressure. 2. While treatment with a relatively low dose of estrogen did not have any significant overall effect on the release of hypothalamic NE it was previously found that synthesis of NE in this region of the brain in estrogen-treated rats occurs only in the late-dark period when the maximum level in blood pressure occurs. 3. Although it is known that NE synthesis takes place in the medial lower brainstem during the late-dark period in estrogen-treated and control rats, no significant release of NE from this region of the brain was found at any of the 3 times sampled, 1 1/2 h following the administration of amphetamine. 4. Estrogen appears to abolish the release of cerebellar NE which occurs at the late-dark phase in control rats. This finding is consistent with the previously reported absence of NE synthesis at late-dark and early-light phases following estrogen administration, and 1 1/2 h after the injection of alpha-methyl-p-tyrosine. 5. No temporal differences in the release of adrenal NE at the sampled times were found, although such changes have been observed to occur in adrenal NE biosynthesis in estrogen-treated rats. 6. Following administration of amphetamine, temporal differences were found in cardiac NE levels at late-dark and mid-light phases in both estrogen-treated and control rats.
