1. The immediate haemodynamic and metabolic effects of intravenous administration of dl-propranolol and d-propranolol were studied in healthy male subjects at rest and during dynamic forearm exercise. The dose of dl-propranolol and of d-propranolol was 0.1 and 1.0 mg/kg body weight respectively.
2. dl-Propranolol reduced heart rate significantly at rest, during forearm exercise and post-exercise, whereas d-propranolol had a lesser effect on heart rate which was significant only at the end of the exercise period. Arterial blood pressure and forearm blood flow were unchanged after either drug.
3. Both drugs reduced the release of lactate from the exercising forearm. Forearm exchange of oxygen, glucose, free fatty acids and triglycerides remained unchanged.
4. The arterial blood glucose concentration increased after d-propranolol, but was unchanged after dl-propranolol. The arterial serum free fatty acid concentration decreased after dl-propranolol, but was not changed after d-propranolol. Arterial concentrations of lactate and triglycerides were not influenced by the drugs.
5. The chronotropic response to β-adrenoceptor blockade appears to be stereoselective, suggesting specific blockade of β-adrenoceptors. Metabolic responses to β-adrenoceptor blockade are difficult to explain in terms of the known adrenoceptor system and may be due to nonspecific actions of β-adrenoceptor antagonists. An exception is inhibition of lipolysis, which is probably mediated via β-adrenoceptors.