The effect of 30 mg./70 kg. of propiomazine on the respiratory response to endogenously accumulated CO2was measured in 5 volunteers and found to have no depressant action. Irregular, sighing respiration was noted, however. Three of 9 volunteers given 30 mg./70 kg. propiomazine became quite hypotensive when suddenly tilted head-up 60 degrees. Approximately one half the subjects became restless after injection of propriomazine. It was concluded that the optimal use of the drug is in conjunction with small doses of narcotics. Propiomazine is pharmacologically very similar to its related compound promethazine in its lack of respiratory depression but positive circulatory depressing action.