Rats were injected i.c.v. with arginine(8)-vasopressin or saline 30 min after they had received an i.p. injection with the catecholamine synthesis inhibitor alpha-methyl-p-tyrosine. Three hours after the administration of vasopressin or saline the noradrenaline content of the hypothalamus, thalamus and medulla oblongata of the vasopressin-treated rats was significantly lower than that of the same parts of saline-treated rats. These results indicate that vasopressin increases nerve impulse flow in noradrenaline neurons in particular brain regions.