Two field experiments were carried out during the two successive seasons of 2005 and 2006 to study the effects of different concentrations of atonik ( 0, 50, 100 and 150 mg/l ) and paclobutrazol ( 0, 30 and 60 mg/l ) as well as their interaction on growth, photosynthetic pigments, yield and its components as well as biochemical constituents of the produced grains of two hybrids of yellow maize. The results showed that there were significant differences between the two yellow maize hybrids; single crosses ( S.C. 155 and S.C. 161 ) in most growth characters at different stages of growth, photosynthetic pigments content in the leaves, yield and its components as well as biochemical constituents of the grains. Spraying atonik with different concentrations at all stages led to significant increases in plant height, flag leaf area, 4^th leaf area, LAI, SLW, CGR, dry weight/plant, photosynthetic pigments content, yield and biochemical constituents of grains. Spraying paclobutrazol up to 30 mg/l caused significant increase in most growth characters, photosynthetic pigments content, yield and its components as well as biochemical constituents in the grains. While spraying with 60 mg/l paclobutrazol led to significant decreases in plant height, flag leaf area, LAI, SLW, CGR at the different stages of growth compared with the control plants. Spraying the two yellow maize hybrids with atonik at 150 mg/l or paclobutrazol at 30 mg/l caused the highest values in most of growth characters. Spraying with 100 and 150 mg/l atonik and paclobutrazol at 30 and 60 mg/l attained significant increases in economic yield and its components. Application of atonik treatments in general had positive effects on crude protein, carbohydrate and carotenoids contents in grains. The highest values was obtained at 30 mg/l on crude protein content and 60 mg/l on total carbohydrate and carotenoids content. Interaction between atonik at 100 and 150 mg/l and paclobutrazol at 30 and 60 mg/l gave the highest increment in the growth characters and chlorophyll (a), (b) and total carotenoids. Application of atonik at 150 mg/l with paclobutrazol at 60 mg/l increased significantly yield and its components, carotenoids content in leaves and total carbohydrate content. Interaction between the two yellow maize hybrid and atonik at 150 mg/l or paclobutrazol at 60 mg/l gave the highest values in most growth characters, photosynthetic pigments content, yield components and carotenoids in produced grains.