Previous research has shown that kernel sink capacity, the potential of maize ( Zea mays L.) kernels to achieve maximal mass, is established during the early stages of kernel development and is a function of the number of endosperm cells and starch granules formed. In this study, the genetic regulation of endosperm cell and starch granule number was investigated. Kernel growth was characterized in four disparate inbred lines (which differed in relative maturity and kernel size) and their reciprocal crosses. Relative maturity affected the pattern of kernel development (i.e., when endosperm cell division and starch granule formation was initiated) but not the maximum number of endosperm or starch granules formed nor the mature kernel mass obtained. Kernel growth rate and mature kernel mass of the large kernel inbreds were nearly twofold higher than that of the small kernel inbreds, regardless of relative maturity. Comparison of F~ kernels from reciprocal crosses revealed a strong maternal influence on mature kernel mass. However, in reciprocal crosses between A619 (large kernel) and W64A (small kernel), the maternal effects on mature kernel mass were due to changes in the number of endosperm cells formed, whereas in reciprocal crosses between Mo17 (large kernel) and B73 (small kernel), it was due to modulation of the number of starch granules. The results confirm that the number of endosperm cells and/or starch granules are good indicators of kernel sink capacity in maize. Moreover, the data show a strong maternal influence on kernel sink capacity and mature kernel mass. However, whether the number of endosperm cells or the number of starch granules is the more important determinant of mature kernel mass is genotype dependent.