ABSTRACT.The influence of hypoxia and hypercapnia on the ventilatory rhythm of the hellgrammite Corydalus cornutus L. (Megaloptera) was studied. In intact animals the frequency of rhythmic retractions and protractions of abdominal gills is increased by hypoxia (10% O2, 90% N2) but no ventilatory response is elicited by hypercapnia (1–5% CO2, 20% O2, 75–79% N2).The ventilatory motor pattern was examined by recording extracellularly from the gill retractor muscle or its efferent nerve. In response to hypoxia (8% 02, 92% N2), there are decreases in the cycle‐time, the interspike interval, and the burst length of the gill retractor motorneuron. In addition, previously quiescent motorneurons associated with gill protraction are recruited.Individual ganglia or small groups of abdominal ganglia can be isolated both from the central ganglionic chain and from the periphery by selective cutting of roots and connectives. When exposed to hypoxia, preparations that include the first abdominal ganglion show characteristic changes in the ventilatory motor pattern similar to those in intact animals. Thus sensitivity to oxygen appears to be located centrally and not peripherally. In small animals (head width9 mm), chains of ganglia lacking abdominal ganglion 1 fail to respond. In larger animals oxygen sensitivity may thus be concentrated in abdominal ganglion 1, whereas in smaller animals the ability to initiate a ventilatory response to hypoxia is distributed among the abdomina