1. Acetylcholine and nicotine were superfused on the ventral medullary surface between the ponto-medullary border and C1 in anaesthetized cats in order to determine the topical distribution of their actions on respiration and circulation. 2. Acetylcholine (10(-4) g . ml-1 = 5.5 . 10(-4) mMol . ml-1) produced an increase in respiration and a lowering of blood pressure. The magnitude and the time course of the responses varied according to the points of superfusion on the surface. 3. Nicotine (10(-4) g . ml-1 = 6.2 . 10(-4) mMol . ml-1) elicited hyperventilation and more often an increase in arterial pressure on unilateral superfusion of the surface. In some cases, however, a drop in blood pressure was also observed. 4. The responsive regions of the surface on which nicotine acted and elicited hyperventilation, bear a close resemblance to the regions responsive to acetylcholine. 5. The topographical distribution of the respiratory effects elicited by the above-mentioned drugs were similar to the distribution of the responses to changes in pH on the ventral medullary surface or to electrical stimulation. 6. Procaine (2 . 10(-2) g . ml-1 = 7.3 . 10(-2) mMol . ml-1) applied bilaterally in the intermediate zone (S) caused profound inhibition of respiration and of arterial pressure. Procaine at this concentration also inhibited respiratory hyperventilation caused by nicotine (10(-4) g . ml-1 = 6.2 . 10(-4) mMol . ml-1) applied to the caudal and rostral areas.