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1. The effects of noradrenaline and isoprenaline on the repolarization phase of the action potential have been studied in the Purkinje fibres of sheep heart, electrically driven at constant rates.

2. Isoprenaline (2, 5 and 8 x 10^-8 g/ml.) increases the slope of phase 2 of repolarization and decreases the plateau length; the resulting decrease in action potential duration is concentration dependent, but not rate dependent.

3. The effect of isoprenaline on the action potential duration is entirely blocked by propranolol (10^-7 g/ml.) and unaffected by phentolamine (5 x 10^-7 g/ml.).

4. In phentolamine (5 x 10^-7 g/ml.) pretreated preparations the response induced by noradrenaline (5 x 10^-8, 1 and 5 x 10^-7 g/ml.) is very similar to that induced by isoprenaline.

5. In propranolol (2·5 x 10^-7 g/ml.) pretreated preparations noradrenaline causes a lengthening of the plateau phase and an increase in action potential duration.

6. The relationship of these results to the presence of both - and -receptors in cardiac Purkinje fibres is discussed.

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