1. The fall in arterial blood pressure with bradycardia that occurs on injection of clonidine into the cerebral ventricles and into the cisterna magna is attributed to an action on ;chemosensitive zones' situated at the ventral surface of the brain stem. This conclusion is based on the following results obtained in cats anaesthetized with pentobarbitone sodium.2. The fall in blood pressure no longer occurs on injection of clonidine (10 to 100 mug) into the cerebral ventricles when the passage of clonidine into the subarachnoid space is prevented by cannulation of the aqueduct. In this condition, the injections produce instead a rise in blood pressure.3. Applied bilaterally, by means of perspex rings, to the ventral surface of the brain stem, clonidine (10 mul of a 50 to 1,000 mug/ml solution placed in each ring) produces a fall in blood pressure with bradycardia, but only when the perspex rings cover the ;chemosensitive zones' from which changes in blood pressure and heart rate are obtained with various drugs.