A plant test for determining the nitrogen (N) fertilizer requirement of tobacco was developed and evaluated in laboratory, pot and field experiments. Sampling procedures for measuring the nitrate-N in petiole and stem sap were examined. In particular, the effects of time of sampling, duration of sample collection, plant age and fertilizer form were determined. Transplants of different tobacco types were grown in plastic bags containing 3.5 kg of soil each. The plants were subjected to five fertilizer rates corresponding to zero (control), low, medium, high and very high fertilization. The different rates were equivalent to 0, 25, 50, 100 and 200 kg N and potassium per hectare, respectively. Phosphorous was applied at a constant rate of 50 kg/ha. Soils were maintained at field capacity moisture. For each treatment, 60 seedlings of each tobacco variety were planted and maintained. Plants were destructively sampled at 30-31, 38-39 and 44-45 days after transplanting to correspond to early, mid and late vegetative stage of growth. At each designated sampling day, the plants were cut at the stem in which a tubing was inserted. The stem was allowed to bleed and the sap was collected using a syringe. Stem sap was analyzed for N while the upper plant parts were weighed, oven-dried and re-weighed. Soil samples were also gathered during the sampling periods. To determine the best time to gather stem sap, field set-ups were sampled at five different sampling times. At each sampling time, five plants were chosen at random and stem sap was collected. Further, an alternative method of sap collection was employed to conform with the standard cultural practices in tobacco production. In this method, the sap was collected from the leaf petiole of maturing tobacco leaves. Analysis of stem and petiole sap was determined using both the cardymeter and the standard laboratory procedures. The utility of the rapid plant test for indicating the N fertilizer sidedressing requirement of tobacco was assessed in the field. Four different initial rates of potassium nitrate (0-66 kg N/ha) and two levels of sidedressing (nil and 22 kg N/ha) were the treatments. In the pot experiment, the volume of stem sap was greater from plants whose stems were cut in the afternoon and the sap collected in the following day. However, sap N concentration was higher from stem sap of plants cut and collected in the same day. Sap N concentration varied with tobacco type. Bright air-cured tobacco had the highest sap N concentration in the stem which generally declined with advancing tobacco growth stage. Higher sap N concentration was observed from plants fertilized with nitrate form. Furthermore, increased stem sap and soil N concentrations were noted with increasing rates of N application. In the field experiment, the concentrations of sap N prior to sidedressing were strongly correlated to extractable N in the soil to a depth of 30 cm. Cured yield was significantly affected by the initial rates of N application but not the sidedressing treatments. Highest yield was obtained with the application of 66 kg N/ha. The critical nitrate concentration in petiole sap was found to be 1700 ppm.