1. Changes in arterial pressure associated with a repertoire of natural behaviour patterns in the rat were examined.

2. A hierarchy of such changes was found. Eating and drinking were associated with higher pressures than grooming and exploration, which in turn were associated with higher pressures than resting.

3. Desynchronized sleep was associated with higher pressures than slow-wave sleep.

4. Lesions of the catecholamine neurons of the A2 region of the medulla did not disrupt the normal hierarchy but resulted in an exaggerated pressor response.

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