This is the first report of estimating conduction velocity (CV) of the slowly conducting somatosensory spinal tracts or the spino-thalamic tract (STT) in man. The CV of the STT was measured by recording somatosensory evoked potentials (SEPs) following CO2 laser stimulation of the hand and foot, which was previously shown to cause pain or heat sensation by activating cutaneous nociceptors and by its ascending signals through A delta fibers and probably STT. When the CV of A delta fibers was assumed to be 10-15 m/sec, the CV of STT was found to be approximately 8-10 m/sec in normal young subjects. It was slightly slower in subjects over 60 years of age. In contrast, the CV of the posterior column, which was calculated based on SEPs following electrical stimulation of the median and posterior tibial nerves, was approximately 50-60 m/sec.