A study has been carried out in awake normotensive (WKY), spontaneously hypertensive (SHR) and renal hypertensive (RHR) rats in order to compare the reactivity of the autonomous nervous system using different indwelling catheterizations. In all cases, a jugular catheter was implanted for i.v. injections. The arterial cannula was implanted in the femoral artery (group 1) or in the right carotid artery (group 2), where the catheter was routed over 20 mm towards the heart, reaching the aortic arch. The plasma catecholamines levels were lower in the RHR than in the WKY or SHR, and whatever the strain, lower in group 2 than in group 1. In the WKY and the RHR, the mean blood pressure was higher in group 2 than in group 1. The tachycardia induced by isoprenaline and the pressor response to noradrenaline were higher in the SHR than in the WKY and in the RHR. By contrast, the reflex bradycardia was lower. Furthermore, the procedure of catheterization could modify the cardiovascular responses induced by adrenergic and cholinergic agents. These results demonstrate that indwelling catheterizations cannot be performed safely in the rat when studying effects on cardiac performance or vascular reactivity.