Mammoth cultivars of flue-cured tobacco flower under short-day photoperiods, thereby increasing the potential number of leaves per plant. Field experiments were conducted in 1991 and 1992 at the Virginia Tech Southern Piedmont Agricultural Experiment Station near Blackstone, Virginia to determine the influence of nitrogen rate on nitrogen use efficiency and its components on two mammoth cultivars and a conventional cultivar. Experiments were also conducted to determine the effects of leaf number, time of topping, and nitrogen rate on yield, quality, and other agronomic characteristics. Nitrogen uptake, utilization, and use efficiencies decreased while nitrogen accumulation increased as nitrogen rates increased from 67 to 112 kg ha⁻¹. Mammoth cultivars were more efficient than the conventional cultivar for nitrogen uptake, utilization, and use. Only NC 27 NF benefits from increasing nitrogen above the recommended rate. Yield increased by 17 percent as the leaf number increased from 20 to 36. In contrast, both agronomic traits and chemical characteristics decreased accordingly. The ratio of nitrogen to nicotine that was unacceptable at leaf numbers above 28. Yield, value, and quality decreased as topping was delayed by seven to 21 days. Yield was reduced by 30 kg per hectare per day when topping was delayed by 21 days after the conventional cultivar reached the bud stage. Leaf lengths and widths of mammoth cultivars were larger than the conventional cultivar. Mammoth cultivars were higher yielding and had better quality at 112 kg ha⁻¹ N when compared to the conventional cultivar, although these differences were not significant. / Master of Science