SummaryArousal responses (AR) to hypercarbia and to hypoxia were ascertained in 25 N-M SIDS infants and 21 control infants in whom ventilatory responses to hypercarbia and hypoxia were also measured. Although the frequency of a positive AR to hypercarbia was not significantly less in the N-M SIDS compared to control group, the overall pattern was a generally absent AR in the lowest hypercarbic ventilatory response slope group progressing to a generally positive AR in the highest hypercarbic response slope group. Among the 25 infants having a positive hypercarbic AR, the Mean (± SEM) PACO2at which arousal occurred was 48.5 ± 1.6 in N-M SIDS versus 42 ± 1.2 mmHg in control infants (p < .001). The overall pattern for hypoxic AR was also a generally absent AR in the lowest hypoxic ventilatory response slope group progressing to a generally positive AR in the highest response slope group. Although the PAO2level at which an AR occurred did not differ in the two groups, a positive hypoxic AR occurred in 76% of the control versus only 29% of the N-M SIDS group (p < .01). In summary, infants with a clinical N-M SIDS history and diminished ventilatory response slopes have as a group a concomitant abnormality in hypercarbic and/or hypoxic arousal.