Effects of morphine (1 mg/kg i.v.) were examined on evoked potential of somatosensory afferent pathways elicited by tooth pulp stimulation in cats. Results were as follows: In central gray (CG) of the midbrain which has a triphasic evoked potential with a short latency, morphine decreased the amplitude. In nucl. lateralis posterior(LP) and nucl. medialis dorsalis(MD) of the thalamus, association relay nuclei, which have a late component followed by a fast component with a relatively short latency, morphine decreased the amplitude of the late component but not that of fast component of the evoked potential. In nucl. centralis lateralis(CL) of the thalamus, intralaminar nuclei, and pre-central association area(PCA) of the cortex, which have monophasic and triphasic evoked potentials with a long latency, respectively, morphine markedly decreased the amplitude of both evoked potentials. In nucl. ventralis posteromedialis(VPM) and somatic sensory area I (SI), lemniscal system, which has biphasic and mutiphasic evoked potentials with a short latency, respectively, morphine had no effect on these evoked potentials. As these depressant effects of morphine on evoked potentials were antagonized by naloxone (0.2mg/kg i.v.), a specific morphine action is suggested.