Abstract: The experiment was carried out on the dog by the classical method of conditioned reflex in a sound-proof room. As unconditioned stimulus, acid was poured in one side of the mouth of the dog; and the salivary secretions of its two gl. parotis were collected. The results show that the unconditioned secrection of the left (or the right) gl. parotis of the dog was determined by the unconditioned stimulus on the same side. After unconditioned stimulation on one side of the mouth, the excitability of the nervous centre on the same side was momentarily heightened. The conditioned reflex which had been elaborated with one-sided reinforcement took place primarily on the same side, this might indicate the differentiation between the activities of the left and the right nervous centres. If the reinforcement of a given conditioned reflex was transfered from one side of the mouth to the other, after a few trials the original one-sided conditioned reflex would gradually extinct, and new conditioned reflex would appear and gradually develop on the other side. In the process of such an alteration, the unconditioned secretion of the left and the right gl. parotis rose and lowered alternatively at its final stage. It seems to the author that this fact might be related to the interaction between the centres of the cerebral cortex.