The effects of once daily dosage of the two cardioselective beta-adrenoceptor blocking agents, atenolol and metoprolol, were studied in 26 patients with primary hypertension. The study was a randomized double-blind cross-over trial with placebo run-in and wash-out. Assessment of effect was performed about 1 and 25 hours after dosing. At rest, both atenolol and metoprolol lowered the blood pressure (BP) and heart rate (HR) compared to placebo. Atenolol induced a more effective BP reduction than metoprolol, especially 25 hours after drug intake. During exercise 1 hour after dosing both drugs reduced BP and HR to a similar extent, whereas 25 hours after dosing atenolol gave a more efficient BP and HR reduction than metoprolol. Our data show that both 100 mg atenolol and 100 mg metoprolol are effective antihypertensive beta-blockers at rest and during exercise, 1 hour after intake. Metoprolol was less effective than atenolol 25 hours after dosing probably due to its shorter plasma half-life, thus implying a twice daily regimen for metoprolol in standard preparation.