The results of the electrical stimulation of the locus coeruleus in curarized cats, and the effects upon the observed cortical responses of 5,6-dihydroxytryptamine or 6-hydroxydopamine injected intracisternally or applied in the subarachnoideal space were studied. The changes were followed on the strychnine spikes produced after the topical application of this drug. Stimulation of the locus coeruleus elicited a response in every area of the cortex explored. The response was complex, showed a phase of depression and a phase of enhancement both mixed in different proportions. The depressant reaction was more marked with lower strengths of stimulation and the enhancement reactions appeared more evident with the stronger stimuli. Both these reactions were blocked by the previous administration of 6-hydroxydopamine and only the depressant one was practically abolished by 5,6-dihydroxytryptamine. It is concluded that a complex interaction exists between the noradrenergic neurons of the locus coeruleus and the serotonergic neurons, probably those of the raphe nuclei. It is suggested that the axons of the noradrenergic neurons in their way to the cortex, give off collaterals that innervate some serotonergic neurons. The monoamines liberated by these axon terminals at the cerebral cortex would act as neurohumoral agents.