1. The concentrations of adrenaline and other catecholamines (noradrenaline and its major metabolite DHPG, dopamine and its major metabolite DOPAC) were measured in the hypothalamus and medulla oblongata of normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) which had received a continuous subcutaneous infusion of clonidine (10 micrograms/kg per h) for 10 days, and also in WKY and SHR rats which were killed 15-18 h after the cessation of the 10-day infusion. 2. The concentrations of adrenaline in the hypothalamus and medulla oblongata/pons of the clonidine treated WKY and SHR rats were not different from their respective controls. However, the adrenaline concentrations in the hypothalamus (but not the medulla oblongata) were significantly decreased in the post-infusion WKY and SHR. 3. These results suggest that hypothalamic adrenergic mechanisms may have a common involvement in the post-clonidine infusion syndromes displayed by the WKY and SHR strains.