Experiments were carried out to determine if endogenous pyrogen-induced fever impairs protective responses of newborn rats to hypoxia. Twenty-seven 5- to 6-day-old conscious rat pups received a subcutaneous injection of 0.20 µg of recombinant rat interleukin-1β (rrIL-1β) per kilogram of body weight to induce fever, or an equal volume of vehicle. They were then either exposed to a single period of hypoxia produced by breathing an anoxic gas mixture (97 % N2-3 % CO2) and their time to last gasp was determined, or they were exposed repeatedly to hypoxia and their ability to autoresuscitate from primary apnoea was determined. Core temperature increased significantly following administration of rrIL-1β but did not change following administration of vehicle (i.e. vehicle, 0.0 ± 0.1 °C; rrIL-1β, 0.7 ± 0.3 °C; P < 0.001) before exposure to hypoxia. IL-1β-induced fever did not alter the time to last gasp when the pups were exposed to a single period of hypoxia or the number of successful autoresuscitations upon repeated exposure to hypoxia. Thus, our data do not support the hypothesis that endogenous pyrogen-induced fever impairs the protective responses in newborns that may prevent death during hypoxia as may occur during single or repeated episodes of prolonged sleep apnoea. Experimental Physiology (2002) 87.1, 17-24.