One way for producers to capture extra value may be to grow maize grain with starch and protein compositions that are tailored for specific end-use markets. However, the genetic and management constraints may be different for the particular grain types (i.e. high starch vs. high protein), and they are likely dictated by the production and utilization of assimilates by the plant. Thus, the objective of this study was to determine the relative importance of the assimilate supply from the source plant and the ability of the grain sink to utilize assimilates on the final composition of the maize kernel. Six intermated B73xMo17 recombinant inbred (IBM) lines which displayed contrasting concentrations of endosperm starch (three high and three low) were grown in the field with high and low N availability (0 or 68 kg N/ acre), and the sink/source  ratio altered by partial ear degraining (45% reduction) at 15 days after pollination (DAP). The instantaneous assimilate supply into the kernels at 19 DAP (for two high and two low starch inbreds) was determined using the agar trap technique. For all inbreds, increases in endosperm starch and protein contents (mg/endosperm) following degraining suggested the assimilate source supply limited grain composition. The greater relative increase in protein content (approx. 40%) than starch (approx. 20%) following degraining, and the increase in grain protein with N supply suggests that N source particularly limits grain protein. The C/N ratio of the assimilate supply at 19 DAP, was directly related to the final kernel composition, and there was an inverse relationship between the concentrations of starch and protein in the mature endosperm (r= 0.87). These data show that inbreds of the IBM lines are source limited due to an inability to uptake N from the soil, and then to transport maximal N and sucrose to developing grains.