1 The effects of noradrenaline upon the cardiovascular system of the rat, anaesthetized with pentobarbitone, have been investigated.2 Noradrenaline produces a dose-dependent increase in mean arterial blood pressure (MABP) which is due entirely to an increase in cardiac output; total peripheral vascular resistance (TPR) remains unchanged.3 Following beta-adrenoceptor blockade the pressor response to infused noradrenaline is enhanced and is now due mainly to an increase in TPR; the increment in cardiac output is reduced.4 After alpha-adrenoceptor blockade the pressor response is greatly reduced; the residual increase in MABP is due solely to an increase in cardiac output.5 After ganglion blockade resting cardiac output and TPR both fall, resulting in a reduction in MABP. The pressor response to noradrenaline is enhanced and is now due to increases in both TPR and in cardiac output.6 The cardiovascular response of the anaesthetized rat to noradrenaline can be explained in terms of classical alpha- and beta-adrenoceptor stimulation by the amine; the unusual form of the response may be due to an effective predominance of beta-adrenoceptor-mediated effects in this species.7 It is suggested that the failure of exogenous noradrenaline to produce a rise in TPR results from a balance between the alpha-adrenoceptor-mediated increase and beta-adrenoceptor-mediated decrease in this variable. However, this proposed balance is lost if resting vasoconstrictor tone is reduced by ganglion blockade.