Nitrate uptake by roots of cowpea (Vigna unguiculata) was measured using (15)NO(3) (-), and the energy cost to the root was estimated by respirometry. Roots of 8-day-old cowpea seedlings respired 0.6 to 0.8 milligram CO(2) per plant per hour for growth and maintenance. Adding 10 millimolar NO(3) (-) to the root medium increased respiration by 20 to 30% during the following 6 hours. This increase was not observed if the shoots were in the dark. Removal of NO(3) (-) from the root medium slowed the increase of root respiration. The ratios of additional respiration to the total nitrogen uptake and reduced nitrogen content in roots were 0.4 gram C per gram N and 2.3 grams C per gram N, respectively. The latter value is close to theoretical estimates of nitrate assimilation, and is similar to estimates of 1 to 4 grams C per gram N for the respiratory cost of symbiotic N(2) fixation.