The pain thresholds, to a cutaneous thermal stimulus and to a tibial pressure stimulus were measured before, during, and after the administration of nitrous oxide (25–30 per cent in oxygen), halothanc (0.5 per cent) and sodium thiopcntal (100–150 mg. in 10 minutes). The thermal pain and the tibial pressure pain thresholds rose with nitrous oxide and halothane. The thermal pain threshold rose with sodium thiopcntal but simultaneously the tibial pressure pain threshold fell. It is deduced that the sensations produced by these two stimuli are neurologically different and that the known differences between the actions of barbiturates and inhalational anesthetic drugs on presynaptic inhibition in the spinal cord can account for this differentiation.