In a previous study in conscious normoxic newborn rats, we found that the strength of the Hering-Breuer reflex (HB reflex) was greater (188%) at high (36°C) than at low (24°C) ambient temperature (T_a; D. Merazzi and J. P. Mortola. Pediatr. Res. 45: 370-376, 1999). We now asked what the effect would be of changes in T_a during hypoxia. Rat pups at 3-4 days of age were studied in a double-chamber airflow plethysmograph. The HB reflex was induced by negative body surface pressures of 5 or 10 cmH₂O and quantified from the inhibition of breathing during maintained lung inflation. Rats were first studied at T_a = 32°C in normoxia, followed by hypoxia (10% O₂ breathing). During hypoxia, oxygen consumption (O₂) averaged 47%, and HB reflex 115%, of the corresponding normoxic values, confirming that in the newborn, differently from the adult, hypoxia does not decrease the strength of the HB reflex. As hypoxia was maintained, lowering T_a to 24°C or increasing it to 36°C, on average, had no significant effects on O₂ and the HB reflex. However, with 5-cmH₂O inflations, the HB reflex during the combined hypoxia and hyperthermia was significantly stronger than in normoxia. We conclude that in conscious newborn rats during normoxia the T_a sensitivity of the HB reflex is largely mediated by the effects of T_a on thermogenesis and O₂; in hypoxia, because thermogenesis is depressed and O₂ varies little with T_a, the HB reflex is T_a independent. The observation that the reflex response to lung inflations during hypoxic hyperthermia can be greater than in normoxia may be of importance in the pathophysiology of apneas during the neonatal period.