1. Plasma renin activity (PRA) and mean blood pressure were studied in conscious rabbits infused with beta-adrenoreceptor antagonists.

2. Oxprenolol and dl-propranolol each significantly reduced PRA and blood pressure, but prindolol, which had a strong blood pressure-lowering effect, increased PRA.

3. When prindolol was given to animals in which PRA and blood pressure had been reduced by dl-propranolol, PRA returned to control values but blood pressure remained low. Thus the increase in PRA caused by prindolol is not mediated by hypotension. These findings, together with the observation that compound H35/25 reduced PRA without altering blood pressure, suggest that the effects of the beta-adrenoreceptor-blocking drugs on blood pressure are unrelated to their effects on renin release.

4. Studies with d-propranolol and with blocking agents with either beta-1 or beta-2 specificity indicated that the effects of beta-adrenoreceptor blockade on renin are directly dependent upon their action on beta-adrenergic receptors, probably of the beta-2 type.

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