The effects of intravenously infused phenylephrine and isoprenaline upon the cardiovascular system of the rat anaesthetized with pentobarbitone, have been investigated. Phenylephrine produces a dose‐dependent rise in mean arterial blood pressure (MABP) that is due mainly to an increase in total peripheral vascular resistance (TPR), though at all doses tested cardiac output was invariably raised. The increase in cardiac output was due in each instance to an increase in stroke volume, heart rate being unchanged. This increase in cardiac output is probably brought about by effects of phenylephrine on the capacitance vessels rather than by an effect on the heart. Evidence is presented to show that the effects of phenylephrine are mediated largely by α‐adrenoceptors, but that β‐adrenoceptors which affect TPR are also stimulated by the amine. Isoprenaline produces a dose‐dependent fall in MABP that is due entirely to a fall in TPR since the cardiac output increases. Unlike phenylephrine, the increase in cardiac output obtained with isoprenaline was achieved by an increase in heart rate while stroke volume remained close to control values. It is contended that the augmented venous return required for the elevated cardiac output results in this case mainly from the isoprenaline‐induced fall in TPR which enhances transfer of blood from arteries to the veins. Evidence is presented to show that the effects of isoprenaline are mediated mainly by β‐adrenoceptors. Under the present experimental conditions the adrenoceptor‐mediated cardiovascular changes are little modified reflexly by the arterial baroreceptors.