1. The effects were studied of morphine and naloxone on the respiratory minute volume and the minute volume stimulated by carbon dioxide in the anaesthetized rabbit. 2. The respiratory minute volume is reduced by morphine (2 mg/kg) to about 60% of control. The effect of morphine is abolished by naloxone (5 mg/kg); the minute volume depressed by morphine transiently becomes larger after naloxone than prior to morphine. Also the minute volume stimulated by carbon dioxide becomes larger after naloxone than that prior to the period of depression produced by morphine. 3. Naloxone (0.5 mg and 5 mg/kg i.v.) enhances the sensitivity of respiration to carbon dioxide by about 21% and 28%, respectively. 0.5 mg/kg naloxone itself had no influence, 5 mg/kg for about 5 min increases basal respiratory minute volume. 4. It is assumed that endogenous opiates modulate respiration and, like exogenously administered opiates, reduce the sensitivity of respiration to carbon dioxide. 5. The hypothesis is discussed that endogenous opiates are involved in regulation of biological functions which are known to be inhibited by opiates and which belong to the "protective system" postulated by O. Schaumann.