Electrical recordings from the rat spinal trigeminal nucleus indicates that activation of single second-order neurons by stimulation of the face may be influenced by prior stimulation of other skin areas (contralateral face, extremities), conditioning and testing stimuli consisting of single shocks through needles inserted in the skin. At conditioning-testing intervals of about 7-15 msec. the test response was facilitated as indicated by an increase in the probability of firing or number of spikes, and by a decrease in latency of the first spike. Blocking (effects opposite to those for facilitation) occurred at longer (up to about 150 msec.) conditioning-testing intervals. The conditioning stimuli could cause spike discharges in these second-order trigeminal neurons, the facilitation and blocking resulting whether or not this occurred. It is hypothesized that the data may result from graded excitatory post-synaptic potentials and their ensuing hyperpolari-zations generated in these cells by the conditioning stimuli.