Both isocapnic and poikilocapnic hypoxia may elicit a biphasic respiratory response, during which an initial ventilatory stimulation is followed by a reduction in breathing and diaphragm (DIA) electrial activity. To ascertain whether during adulthood other respiratory muscles have biphasic hypoxic responses similar to the DIA, in nine anesthetized cats electromyograms (EMG) were recorded from the DIA, genioglossus (GG), and triangularis sterni (TS) (n = 7) muscles during poikilocapnic hypoxia. DIA and GG EMG started at 60 ± 4 and 9 ± 3 units, respectively, during O2 breathing, increased to a maximum of 100 units during the 10-min hypoxic stimulus, and subsequently declined to 81 ± 6 and 58 ± 12 units, respectively, by the end of 10 min of hypoxia. The time course of the increase and subsequent decline was similar for the DIA and GG and for GG activity during both inspiration and expiration. Furthermore the degree to which GG EMG declined after reaching its peak activity level correlated with the magnitude of the respective decline in DIA EMG (r = 0.79, P < 0.02). The TS, in contrast, was maximally active either during O2 breathing or very early during hypoxia, and its activity declined progressively thereafter (to 13 ± 6% of its peak value at the end of 10 min of hypoxia). The degree to which TS EMG declined did not correlate with the degree to which DIA or GG EMG declined. These data indicate that both quantitative and qualitative differences are present among different respiratory muscles in the degree to which they initiate and sustain a stimulatory response to poikilocapnic hypoxia, suggesting that complex mechanisms have a role in determining respiratory motor output during hypoxia.