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To measure the contributions of parental inbreds per se and heterosis to the genetic gain of single- cross maize (Zea mays L.) hybrids, yield trials of 29 historically important hybrids used in China during 1964 through 2001 and their parental inbreds were conducted. Mean rate of genetic gain for single-cross hybrids when measured across all locations was 55 kg ha-1 yr-1; heterosis contributed 37 kg ha-1 yr-1 (67%) of yield gain. Inbred yields at stressed locations were exceptionally poor and may have inflated measurements of heterosis. At unstressed locations, heterosis contributed 5 kg ha-1 yr-1 (8%) and 16 kg ha-1 yr-1 (39%) or 10 kg ha-1 yr-1 (19%) when locations were combined. For individual characteristics, the highest percent heterosis was for kernel weight per ear (58.6%), corresponding to and emphasizing the historic goal in China of increasing yield on an individual plant rather than on a unit area basis. There are opportunities to further improve the productivity of Chinese maize by increasing inbred parent yields, increasing tolerances to stresses associated with higher planting densities, and also increasing the contribution of heterosis through optimal assignation of inbreds into complementary heterotic groups. [ABSTRACT FROM AUTHOR]