Failure to autoresuscitate from apnea has been suggested to play a role in sudden infant death. Little is known, however, about factors that influence the gasping and heart rate response to severe hypoxia which are fundamental to successful autoresuscitation in the newborn. The present experiments were carried out on 184 rat pups to investigate the influence of the parasympathetic nervous system as well as adenosine in mediating the profound bradycardia that occurs with the onset of hypoxic-induced primary apnea and in modulating hypoxic gasping. On days 1 to 2, 5 to 6 and 10 to 11 postpartum and following bilateral cervical vagotomy (VAG) or administration of a selective adenosine A1 receptor antagonist (DPCPX), each pup was exposed to a single period of severe hypoxia produced by breathing an anoxic gas mixture (97% N₂ & 3% CO₂). Exposure to severe hypoxia resulted in an age-dependent decrease in heart rate (p<0.001) -- accentuated with increasing postnatal age -- that was attenuated in all age groups by DPCPX but not by VAG. Furthermore, DPCPX but not VAG decreased the time to last gasp but increased the total number of gasps in the 1 to 2 and 5 to 6 day-old pups but not in the 10 to 11 day-old pups during exposure to severe hypoxia. Thus, our data provide evidence that adenosine acting via adenosine A1-receptors plays a role in modulating hypoxic gasping and in mediating the profound bradycardia, which occurs coincident with hypoxic-induced primary apnea in rats during early postnatal life.