The neuropeptide, pituitary adenylate cyclase-activating polypeptide (PACAP), has emerged as a principal and rate-limiting regulator of physiological stress responses in adult rodents and has been implicated in SIDS (Sudden Infant Death Syndrome). Recent studies show that PACAP plays a role in neonatal cardiorespiratory responses to hypoxia, hypercapnia and hypothermia, but not hyperthermia, which is often associated with SIDS. Here we tested the hypothesis that, consistent with a role in SIDS, PACAP is involved in regulating the neonatal cardiorespiratory responses to severe heat. To address this, we studied the cardiorespiratory physiology of conscious neonatal PACAP-null and wild-type mice at ambient temperatures of 32°C (baseline) and 40°C (heat stress), using head-out plethysmography and surface ECG electrodes. We also assessed body surface temperature, to give an indication of cutaneous heat loss. Our results show that wild-type neonatal mice respond to heat stress by increasing ventilation (P=0.007) and associated expired CO₂ (P=0.041), heart rate (P<0.001) and cutaneous heat loss (P<0.001). In PACAP-null neonates, this heat response is impaired, as indicated by a decrease in ventilation (P=0.04) and associated expired CO₂ (P=0.006), and a blunted increase in heart rate (P=0.001) and cutaneous heat loss (P=0.0002). In addition, heart rate variability at baseline in PACAP-null neonates was lower than in wild-type controls (P<0.01). These results suggest that during heat stress, PACAP is important for neonatal cardiorespiratory responses that help regulate body temperature. Abnormal PACAP regulation could, therefore, contribute to neonatal disorders in which the autonomic response to heat stress is impaired, such as SIDS.