Juvenile mortalities in large mammals are usually associated with environmental extremes, but the basis for this vulnerability is often unclear. Because of their high surface area to volume ratio, juveniles are expected to suffer greater thermal stresses relative to adults. Coping with thermal stress requires the ventilatory system to accommodate increases in oxygen demand and respiratory water loss at thermal extremes. Because juveniles are smaller than adults, these demands may set up different constraints on their ventilatory system. Using red kangaroos (Macropus rufus Desmarest), an arid zone species, we compared the ventilatory capabilities of juveniles and adults at thermoneutral (25 degrees C) and extreme (-5 degrees C and 45 degrees C) ambient temperatures. We used an allometry to compare juvenile to adult ventilation, using predicted body mass scaling exponents for oxygen consumption 0.75), respiration rate (-0.25), tidal volume (1.0), ventilation rate (0.75) and oxygen extraction (0.0). At ambient 25 degrees C, the juveniles resting metabolic rate was 1.6 times that of the mature females (ml min(-1) kg(-0.75)), accommodated by significantly higher levels of oxygen extraction of 21.4 +/- 1.8% versus 16.6 +/- 1.9% ( P< 0.05). At thermal extremes, juveniles showed typical mammalian responses in their ventilation, mirrored by that of adults, including higher metabolic and ventilation rates at ambient -5 degrees C and shallow panting at 45 degrees C. However, at thermal extremes the juvenile kangaroos needed to work harder than adults to maintain their body temperature, with higher rates of ventilation at ambient -5 degrees C and 45 degrees C, accomplished via larger breaths at -5 degrees C and higher respiratory rates at 45 degrees C.