Argon, nitrogen, nitrous oxide were administered hyperbarically in doses (atmosphere) that caused loss of righting reflex (LORR). Nitrous oxide requires pressure somewhat less than two atmospheres, eighteen atmospheres were required for argon and thirty-six atmospheres roughly for nitrogen all in 0.5 atmospheres oxygen. Loss of righting reflex was assessed by using a rolling cage method of Wilson and Miller. Since nitrogen is the least liposoluble and nitrous oxide the most liposoluble of these three gases, greater pressures were needed for nitrogen to attain sufficient concentration in the membrane for anesthesia. Due to the low lipid solubility (1.4), nitrous oxide was administered hyperbarically at a compression rate of less than 0.5 atm/min at chamber temperature of 86 degrees plus or minus 2 degrees. Body temperatures were measured by minimitter transmitters. Two types of transmitters: an AM frequency and an FM frequency were used; a comparison of the two systems were made. The ED50 (atmospheres) required to produce a given score on the LORR were determined for each strain or line of mice. This ED50 value was determined for the Hot and Cold selection lines which have been specifically bred to differ as much as possible in a hypothermic response to acute doses of ethanol. These experiments demonstrate quite clearly a degree of commonality exists among CNS depressants with regard to anesthesia, loss of righting reflex and hypothermia.