Male albino rats received 10 µg of p-hydroxynor-ephedrine (PONE) 24 h prior to the injection of 20 µCi of 3H-tyrosine. Animals were sacrificed 15 and 30 min later and the brains analyzed for 3H and endogenous norepinephrine (NE), dopamine (DA) and tyrosine. It was observed that PONE increased the rate of disappearance of 3H-catecholamines. This effect is apparently not related to a change in tyrosine hydroxylase activity, which was found to increase, but rather to an increase in the rate of metabolism of 3H-catecholamines.