The mechanism of the effect of mild calcium deficiency on nitrogen metabolism of the symbiotic plant was studied from the distribution of calcium and of nitrogen and carbohydrate fractions in plant organs.Nitrogen concentrations of all plant organs decreased with calcium deficiency. Addition of either a nitrogen or a calcium salt increased nitrogen concentrations. For roots as well as whole plants the effects of one salt were largely replaced by the other. These effects establish that calcium deficiency decreased the supply of fixed nitrogen from nodules to other organs. As weight of nodules was independent of calcium it follows that nodular efficiency was impaired.Since nitrogen concentrations of nodules decreased rather than increased with calcium deficiency it is suggested that calcium deficiency interferes with the rate of reduction of nitrogen in the nodule rather than with the export of reduced nitrogen. Distribution of alpha-amino, amide, and ammonium nitrogen are consistent with this suggestion.Nitrogen fixation was not limited by translocation of carbohydrates to nodules as calcium deficiency had little effect on the concentration of soluble carbohydrates and actually increased the concentration of starch in nodules.Calcium deficiency depressed the calcium content of nodules so that nitrogen fixation may have been impaired by inadequate calcium for nodular structure or metabolism.