The nutrient artery passes through the nutrient foramen on the shaft of the femur and supplies more than one-half of total blood flow to the bone. Assuming that the size of the nutrient foramen correlates with the size of the nutrient artery, an index of blood flow rate (Qi) can be calculated from nutrient foramen dimensions. Interspecific Qi is proportional to locomotor activity levels in adult mammals, birds and reptiles. However, no studies have yet estimated intraspecific Qi to test for the effects of growth and locomotor development on bone blood flow requirements. This study uses micro-CT and medical CT scanning to measure femoral dimensions and foramen radius to calculate femoral Qi across in-pouch and post-pouch life stages of western grey kangaroos Macropus fuliginosus weighing 5.7 g to 70.5 kg and representing a 12,350-fold range in body mass. A biphasic scaling relationship for Qi on body mass is evident (breakpoint at ca. 1-5 kg body mass right before permanent pouch exit), with a steep exponent of 0.96±0.09 (95% CI) across the in-pouch life stage and an exponent of -0.59±0.90 across the post-pouch life stage. In-pouch joeys show Qi values 50 to 100-times higher than adult diprotodont marsupials of the same body mass, but gradually converge with them as post-pouch adults. Bone modelling during growth appears to be the main determinant for femoral bone blood flow during in-pouch development, whereas bone remodeling for micro-fracture repair due to locomotion gradually becomes the main determinant when kangaroos leave the pouch and become more active.