Zea mays grown with high levels of N fertilizer transports more sucrose into kernels than with low N. Sucrose translocation was greatest in genotypes with the highest capacity to deposit nitrogenous compounds as zein and glutelin in the kernel. These two proteins combined contain about 80% of the total N in the kernel and about 60% of the total N in the plant at maturity. They appear to serve as a functional N sink for the deposition of nitrogenous compounds. As the N sink capacity increases with additional available N fertilizer, more sucrose is transported into the kernel, resulting in increased kernel weight and grain yield. Zein functions as a more dynamic N sink than glutelin because the synthesis of zein is readily manipulated by N fertilization and genetic means. Increases in N deposition in the normal endosperm induced by N fertilizer are confined primarily to zein. Early termination of zein accumulation in the opaque-2 mutant results in a reduction of sucrose movement into kernels. By using plants heterozygous for normal and opaque-2 in these studies, interplant variability was eliminated and the hypothesis relating the kernel N sink capacity to productivity was strengthened.