The effects of N source (6 mm nitrogen as NO(3) (-) or urea) and tungstate (0, 100, 200, 300, and 400 mum Na(2) WO(4)) on nitrate metabolism, nodulation, and growth of soybean (Glycine max [L.] Merr.) plants were evaluated. Nitrate reductase activity and, to a lesser extent, NO(3) (-) content of leaf tissue decreased with the addition of tungstate to the nutrient growth medium. Concomitantly, nodule mass and acetylene reduction activity of NO(3) (-)-grown plants increased with addition of tungstate to the nutrient solution. In contrast, nodule mass and acetylene reduction activity of urea-grown plants decreased with increased nutrient tungstate levels. The acetylene reduction activity of nodulated roots of NO(3) (-)-grown plants was less than 10% of the activity of nodulated roots of urea-grown plants when no tungstate was added. At 300 and 400 mum tungstate levels, acetylene reduction activity of nodulated roots of NO(3) (-)-grown plants exceeded the activity of comparable urea-grown plants.Addition of tungstate to the nutrient solution decreased plant growth, regardless of the N source, although the effect was more pronounced with NO(3) (-) nutrition. The increased nodulation and decreased nitrate reductase activity noted with plants grown in the presence of tungstate and a high (6 mm) external supply of NO(3) (-) suggests that NO(3) (-) does not directly inhibit nodulation but rather affects nodulation indirectly through subsequent metabolism of NO(3) (-).