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Tolerance, manifested by a diminished electroencephalographic response at cortical and subcortical recording sites, was found in rats subjected to repeated systemic injections of morphine sulfate. Reversal of tolerance to morphine resulted from destruction of the medial thalamus.

1. The effect of electrical stimulation of the motor nerve supplying the whiskers on the activity of single cells in the vibrissal region of the ventrobasal complex of the thalamus has been studied in rats under urethane anaesthesia.2. The stimulation caused protraction of the ipsilateral whiskers. 60% of the cells which fired to mechanical movements of the whiskers were found to respond to this electrical stimulus with 1-2 impulses at short latency (average 7.7 msec), provided the stimulus was sufficient to move the whiskers.3. When the moving whiskers hit a barrier, 92% of the cells responded to the stimulus. The most effective position of the barrier was in front of the whiskers, although other positions often produced a response as well. Static displacement of the whiskers, particularly in the forward direction, could abolish the response or increase its latency.4. The following-frequencies for these cells were 5-10 stimuli/sec. Combinations of electrical stimuli with mechanical ramp movements of the whiskers showed that similar recovery times followed both types of stimuli.5. These results are compared with those reported from studies in the afferent nerve fibres after electrical stimulation of the motor nerve and also with responses in the thalamus following mechanical movements of the whiskers. The possible importance of the latency of these sensory responses is considered.