Using red kangaroos Macropus rufus Desmarest, a large (>20 kg) marsupial herbivore, we compared the digestive capabilities of juveniles with those of mature, non-lactating females on high-quality forage (chopped lucerne Medicago sativa hay) of 43 +/- 1% neutral-detergent fibre (NDF) and poorer quality, high-fibre forage (chopped oaten Avena sativa hay) of 64 +/- 1% NDF. On chopped lucerne apparent dry matter (DM) digestibilities by young-at-foot (YAF) red kangaroos (an age that would normally be taking some milk from their mother), weaned juveniles and mature females were similar (55-59%). On chopped oaten hay apparent DM digestibility was lower in the YAF (35.9 +/- 2.3%) followed by weaned (43.4 +/- 2.8%) and mature females (44.6 +/- 1%). The digestion of NDF and its components (mainly cellulose and hemicellulose) was lowest among the YAF followed by weaned and then mature females. The YAF and weaned kangaroos could not sustain growth on the poor-quality diet, and appeared to be at or near maximal gut fill on both forages; the values being 114-122 g DM for YAF and 151-159 g DM for weaned kangaroos. Mean retention times (MRT) of particle and solute markers were significantly longer for the YAF and weaned kangaroos on oaten hay than on lucerne hay, and DM intake (g d(-1)) was similar to 50% lower on the oaten hay. In contrast, solute and particle MRTs in the mature females were not significantly affected by diet; they maintained DM intakes by increasing DM gut fill from 264 +/- 24 g on chopped lucerne to 427 +/- 26 g DM on chopped oaten hay. Clearly, the mature female kangaroos did not maximise gut fill on the high-quality forage, presumably as a consequence of their proportionally lower energy requirements compared with still-growing juveniles. Overall, we have provided the first mechanistic link between the physiological constraints faced by juvenile red kangaroos in relation to their drought-related mortalities, rainfall and forage quality.