The in vitro sensitivity of the rat aorta to the vasoconstrictor effect of noradrenaline was found to vary according to the time the animals were killed, with a minimum at 10:00 h and a maximum at 16:00 h. This ultradian rhythm was not influenced by the presence of the endothelium but was modified by some, as yet unidentified, circulating factor(s). The results also strongly suggested an ultradian rhythm of in vitro sensitivity to the /3-adrenoceptor agonists, salbutamol and isoprenaline. This rhythm appears to be opposite to that for the sensitivity of aortic a-adrenoceptors to stimulation. In conclusion, our results demonstrate that the regulation of aortic tone follows an ultradian rhythm.