Portable electrodes for rapid quantitative determination of nitrate-N (NO3-N) in plant sap have recently been developed, thus enabling a quick assessment of the crop nitrogen (N) status. The intent of this study was to provide diagnostic criteria necessary for the interpretation of petiole sap NO3-N concentrations for potato (Solanum tuberosum L.). Specific objectives were to characterize the relationship between potato petiole sap NO3-N concentration and petiole NO3-N determined by conventional dry weight analysis, and to establish NO3-N sufficiency ranges (NSR) for irrigated potatoes based on sap analyses associated with maximum and 85-95% of maximum yield. A four-year experiment was conducted on a loamy sand soil (Udic haploboroll) at the Sand Plain Research Farm, Becker, Minn. Ten treatments consisting of various rates and times of N application were evaluated. Recently matured leaves were collected every 10 to 14 days, and petioles were split for sap NO3 and dry tissue NO3 analysis. Two portable NO3 electrodes, the Cardy and Hach meters, designed for quick tests, and a fixed laboratory method, the Wescan, were used to determine sap NO3-N concentrations. Dry petiole NO3-N and petiole sap NO3-N were linearly and significantly related, with regression coefficients r2=0.91,0.92, and 0.93 for the Cardy, Hach, and Wescan methods, respectively. Nitrate-N sufficiency ranges determined with the Cardy were 50 mg L-1 higher than those determined with the Hach. During the vegetative/tuber initiation stage (15-30 days after emergence), NSR were 1350-1450, and 1300-1400 mg L-1 for the Cardy and Hach, respectively. As the season progressed, the NSR decreased. During the tuber bulking period (30-75 days after emergence), NSR were 600-1350 mg L-2 for the Cardy and 550-1300 mg L-1 for the Hach. At maturity, NSR were less than 600 and 550 mg L-1 for the same respective electrodes. Petiole sap NO3-N determined by either the Cardy or Hach methods can be a quick and useful tool to assess potato N status.