Oophorectomy accelerates the turnover of 3H-norepinephrine (3H-NE) in rat brain, but does not decrease endogenous brain norepinephrine content. If ovariectomized rats are treated with a given dose of (α-MPT), an inhibitor of norepinephrine synthesis, brain norepinephrine α-Methyl para-tyrosine levels do not fall to the same extent as in unoperated animals given the same dose of α-MPT, even though the turnover of brain 3H-NE continues to be accelerated. These data indicate that ovariectomy increases brain norepinephrine synthesis, and also decreases the sensitivity of rats to inhibitors of norepinephrine synthesis such as α-MPT. Ovariectomy or treatment with follicle-stimulating hormone (FSH) also increases the accumulation of 3H-catechols (predominantly 3H-NE) in rat brain, following the intraperitoneal administration of 3H-tyrosine.