The electrophysiological study using cats was performed to determine whether or not noradrenaline derived from the locus coeruleus (LC) may act as an inhibitory transmitter or modulator on the auditory and vestibular transmission in the primary relay nuclei. The effects of LC conditioning stimulation on neuron activities in the dorsal cochlear nucleus (DCN) and lateral vestibular nucleus (LVN) were compared with those in the spinal trigeminal nucleus, where conditioning stimuli applied to the LC preceding the test stimulus to the trigeminal nerve markedly reduced the height of postsynaptic field potential and the orthodromic spike generation of relay neurons and type B interneurons. In the LVN, P, N1 and N2 waves of field potential and orthodromic spike generation produced by VIII th cranial nerve stimulation in monoand polysynaptic neurons remained unaffected with LC conditioning stimulation. In the DCN, the height of polysynaptic field potential and orthodromic spike generation in the neurons with long latency elicited by VIII th cranial nerve stimulation were significantly inhibited by LC conditioning stimulation. These inhibitory effects of LC conditioning stimulation on the DCN neurons were not obtained in the animals pretreated with reserpine, however, subsequent administration of noradrenaline into the lateral ventricle produced an inhibition of the spike generation with LC conditioning. Histochemical study demonstrated that nerve terminals (varicosities) with a catecholamine fluorescence were observed in the DCN of cats, but the terminals did not found in the LVN. These results indicate that the inhibitory effects of LC conditioning stimulation are produced in the areas where exist the noradrenaline terminals. It is strongly suggested that noradrenaline derived from the LC acts as an inhibitory transmitter or modulator on the auditory transmission in the DCN, however, the LVN neurons are not regulated with noradrenaline from the LC.