1. Plasma ammonia and heart rate were measured in normal volunteers before, during and after an exercise test following administration of placebo, propranolol, pindolol, metoprolol and nadolol.

2. Small changes in plasma ammonia occurred during exercise after placebo. However, all four β-adrenoceptor antagonists produced considerably greater rises. This effect was most marked after propranolol with the response to pindolol, metoprolol and nadolol being similar and intermediate between placebo and propranolol.

3. Resting heart rate was reduced by all the β-adrenoceptor antagonists except pindolol. Exercise induced changes in heart rate were significantly reduced to a similar degree by all the β-adrenoceptor antagonists.

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