The ratio of nitrate/ammonium in the soil decreases from a high level in the early stages of old field succession to a low level in the climax in the tall grass prairie region of central Oklahoma. Characteristic species from the pioneer and climax stages were grown in culture solutions where the total nitrogen content was held constant, but the form varied from nitrate only to ammonium only. The nitrate reductase activity of leaves and roots was measured in vivo. The results indicated that four pioneer species had relatively high levels of enzyme activity while four climax species had relatively low levels. Nitrate reductase activity was consistently higher in either the leaves or the roots of a species than in the other organ regardless of the nitrogen treatment. Five species had their highest enzyme activity in the leaves and three species had their highest activity in the roots. Enzyme activity generally decreased with decreasing nitrate concentrations, but seven cases were found where there were no significant differences in enzyme activity between any of the nitrogen treatments. All eight species had their highest shoot dry weight on the ammonium only treatment. The conclusion is that the successional position of a species is correlated to the physiological adaptation to the soil nitrogen form.