We have analyzed seedling growth of maize F₁ hybrids, CO₂ evolution from kernels and glucose metabolism in embryonic cells during the germination process, and compared these factors with equivalent factors for their parental inbred lines. The heterotic F₁ hybrid (Oh545×W22) germinates and grows faster than parental inbred lines. The evolution rate of CO₂ from germinating kernels was also higher in the heterotic F₁ hybrid. However, these trend were not seen in the non-heterotic F₁ hybrid (Oh545×Oh45). Glucose metabolism in the embryonic cells was evaluated by in vivo labelling of CO₂ and cellular components with [U-¹⁴C] glucose. In the heterotic F₁ hybrid, the assimilation rate of radioactivities from [U-¹⁴C] glucose into amino acids, proteins, nucleic acids, organic acids and CO₂ was significantly higher than that of the parental inbred lines after 24 hrs of germination. These results show that quick activation of metabolic function in the embryo after the onset of water uptake is due to the heterotic F₁ hybrid genotype, and is one of the key factors in the mechanism by which the heterotic F₁ hybrid expresses its hybrid vigor in the germinating process.