Five sugarcane varieties CAC 5711, Co 785, F 156, Phil 56226, and Phil 62120 were grown under field conditions at different fortility levels to determine the in vivo nitrate reductase activity (NRA), total TVD leaf nitrogen content, and specific leaf weight (SLW), at 5,6,7, and 8 months after planting. These data were studied in relation to sugar yield and its components. NRA increased with increasing fertility levels and decreased with increasing plant age. The activity is highest at six months after planting or two months after the final application of fertilizer N.