Nitrate transport into nodulated plants of soybean (Glycine max), cowpea (Vigna unguiculata) and faba bean (Vicia faba) was investigated. Nitrate entering the root system of soybean did not pass out of the vascular system into nodular tissues in detectable quantities. On the other hand, nitrate could pass from soil through the outer surface of nodules but did not penetrate the infected tissue. Similarly, nitrate was restricted to cortical tissues of cowpea and faba bean. Thus, nitrate cannot inhibit nitrogen fixation as a result of reduction to nitrite by nitrate reductase within the bacteroid zone. These results are, however, consistent with an effect of nitrate on an oxygen diffusion resistance located in the nodule cortex. Unlike nitrate, measurable quantities of 45calcium were transported via the xylem into infected and cortical tissues of soybean nodules: it also passed from the soil into the free space of the nodule cortex.