Exposure of mature, nodulated plants of white clover (Trifolium repens) cv. Blanca to 330 mg dm−3 NO3-N for 8 d caused nitrogenase activity per plant to decrease by 80%. Total nodulated root respiration was not significantly affected but analysis of its components showed an 81% decrease in nitrogenase-linked respiration and a 340% increase in growth and maintenance respiration. Carbon costs of nitrogenase activity (mol CO2 respired per mol C2H4 produced) increased by 45% over the exposure period. Sucrose content of the nodules decreased, but the pattern of decrease did not correlate with that of nitrogenase activity. The oxygen diffusion resistance of the nodules was increased by a factor of five. Characterization of this resistance increase suggests an abnormal modification of the diffusion barrier and it is concluded that alteration in the oxygen supply to the bacteroids is involved in the effect of nitrate on nitrogenase activity.