1 The effect of once daily nadolol therapy in sixteen ambulant patients with essential hypertension has been closely assessed during exercise and over 24 h by continuous intra-arterial recording. 2 The drug was well tolerated and showed similar efficacy to other beta-adrenoceptor blocking agents. Whilst blood pressure reduction was observed throughout the whole day, it was not uniform and lost significance during the morning period when blood pressure levels were highest. 3 This provides further evidence that the antihypertensive action of a beta-adrenoceptor drug over 24 h cannot be predicted from its plasma half-life which, with regard to nadolol, is up to 24 h. 4 An explanation for the loss of blood pressure control during the morning may be that the rapid rise in blood pressure leading to the peak levels at this time may be mediated through alpha- rather than beta-adrenergic receptors at the periphery.