The spontaneous discharge of 30 spinothalamic tract (STT) neurons in the lumbosacral spinal cord of anesthetized monkeys was studied. Interval, correlations, and spectral analyses were performed. Three patterns of discharge were found; these were referred to as the SP1, SP2, and SP3 patterns. The SP1 group had moderately regular discharge trains that were devoid of short-interval spike bursts. The SP2 group had spike trains dominated by short-interval bursts without evidence of low-frequency rhythmicity. The SP3 group had spike trains with features of both the SP1 and SP2 groups. Some correlations were found between the mean discharge rate and stimulus-response classes previously defined by our group (25). Correlations were deduced from a parent data set of 221 STT neurons. Type 1 neurons, which were driven primarily by tactile afferents, were found to have significantly lower mean rates than other 'within-neuron' groups. On the other hand, type C neurons, which had strong input from afferents signalling pressure and noxious stimuli were found to have significantly higher mean rates than all other 'across-neuron' classes. A weak relationship was found between the pattern of discharge and the within-neuron stimulus-response classification. Neurons with a largely tactile coding orientation (types 1 and 2) were most frequently of the SP1 and SP2 classes, whereas neurons with a more prominent nociceptive input (types 3 and 4) were most frequently of the SP2 and SP3 classed. No relationship was apparent between discharge pattern and the across-neuron classes.