Zinc ammonia acetate (ZAA) is marked as an agricultural additive (ACA) and has been shown to increase the yield of corn (Zea mays L.). However, the response is sometime variable and the effective components and the mode of action are not known. To address these issues, the effect of four concentrations (0, 0.85, 8.5, and 85 ppm) of ZAA on laboratory germination and early seedling growth of three maize genotypes were evaluated. The ZAA at 0.85 and 8.5 ppm tended to increase the total seminal root length, and shoot and root dry weight of seedling in all genotypes. Hydroponic studies revealed that plant height and whole plant dry weight of 2-week-old seedlings tended to be increased with 0.85 and 8.5 ppm of ZAA in all genotypes compared to the control. The number of seminal and lateral root for all genotypes, and seminal root total length for two genotypes were significantly increased by ZAA at both 0.85 or 8.5 ppm. However, the high root:shoot ratio observed for ZAA-treated seedlings was not due to increased root growth over shoot growth but resulted from a greater reduction in shoot growth at higher ZAA concentrations. The boron (B) content of the shoots of all genotypes had a positive linear response to increasing ZAA concentrations. Both 0.85 and 8.5 ppm of ZAA increased copper (Cu) and iron (Fe) contents of all genotypes. The calcium (Ca), potassium (K), magnesium (Mg), and phosphorus (P) concentrations were not significantly affected by ZAA treatments, although there was a trend toward an increase for some of these elements at both 0.85 and 8.5 ppm of ZAA. In a greenhouse test, plant height, left area, and shoot dry weight of 3-week-old seedlings of all genotypes were increased by ZAA at 0.85 and 8.5 ppm. Root volume was increased at both 0.85 and 8.5 ppm. Root volume was increased at both 0.85 and 8.5 ppm for one genotype. Considering the extremely low concentration of ZAA required to induce a plant response (0.85 or 8.5 ppm), some possible modes of action of ZAA are discussed.