Blood pressure effects obtained by drugs applied to the ventral surface of the brain stem.

1. In cats anaesthetized with pentobarbitone sodium the effect on arterial blood pressure was examined of substances applied bilaterally to the exposed ventral surface of the brain stem by means of Perspex rings placed lateral to the pyramids and caudal to the trapezoid bodies. Routinely, atropine methyl nitrate, which does not pass the blood-brain barrier, was injected I.V.2. The cholinomimetic substances carbachol and physostigmine, and the amino acids glycine and GABA, caused a fall in arterial blood pressure.3. Atropine produced a small but definite rise in arterial blood pressure, antagonized the depressor effects of the cholinomimetic substances, but not those of the amino acids.4. Strychnine, leptazol and tubocurarine, caused a rise in arterial blood pressure.5. The depressor and pressor effects are due to changes in vasomotor tone. They are central effects brought about by penetration of the substances into the brain tissue from the ventral surface of the brain stem. They are not due to their absorption into the blood stream.6. The depressor effects of the cholinomimetic substances may imitate the action of cholinergic neurones, and those of the amino acids that of central inhibitory neurones ending on cells near the ventral surface of the brain stem and exerting an inhibitory influence on vasomotor tone. The pressor effects of strychnine and tubocurarine may in part result from ;disinhibition', i.e. from an antagonistic action produced by these drugs on the amino acids released from the central inhibitory neurones.