The newborn rabbit responds to acute anoxia, as a result of breathing nitrogen, with successive periods of dyspnoea, primary apnoea, gasping and terminal apnoea. Pethidine caused an increase in the period of primary apnoea and a decrease in the duration and rate of gasping. When nalorphine was combined with pethidine the period of primary apnoea was still increased although the duration and number of gasps were restored to control values. Naloxone, in contrast, acted as a mild respiratory stimulant, shown as a longer phase of dyspnoea. Also it completely abolished the respiratory depression produced by pethidine. Naloxone may be preferable to nalorphine as a drug to reverse the effects of pethidine immediately after birth.