Isotopic fractionation associated with N(2) fixation and NO(3) (-) uptake by plants are relevant to the accuracy of estimates of N(2) fixation based on differences in the natural abundance of (15)N between N(2) fixing and nonfixing plants. The isotope effect on N(2) fixation by soybeans (Glycine max [L] Merrill, variety Harosoy) and red clover (Trifolium pratense [L]) was determined from the difference in (15)N abundance between atmospheric N(2) and the total N of plants grown hydroponically with N-free nutrient solution. In soybeans the isotope effect was found to be +0.98 +/- 0.18 per thousand (beta = 0.99902). In clover the isotope effect was +1.88 +/- 0.14 per thousand (beta = 0.99812). The magnitude of these inverse isotope effects is small. However, they would lead to an underestimation of the amount of N(2) fixed, since the N of atmospheric origin which finally appears in the plant is made richer in (15)N by the inverse isotope effects than is atmospheric N(2), and, to that degree, is attributed to soil-derived N in the calculation.Isotopic fractionation associated with NO(3) (-) uptake by plants does not have a critical effect on estimates of N(2) fixation which are based on natural abundance of (15)N since the (15)N abundance of soil-derived N in plants is measured directly (i.e. after the N has undergone fractionation). Nevertheless, such fractionation is of some interest from the point of view of deciding upon the most appropriate sampling time. The isotope effect on NO(3) (-) uptake by a nonnodulating isoline of soybeans (variety Harosoy), marigold (Tagetes erecta [L]) and ryegrass (Lolium perenne [L]) was estimated from the difference between the (15)N abundance of the total N of plants grown hydroponically and that of NO(3) (-) supplied in the medium. It was found to be about -5 per thousand (beta = approximately 1.005).