Biological fixation and transformation of nitrogen was studied in impoundments in central Oklahoma using the acetylene reduction method and 15N techniques. Nitrogenase activity, probably microbial, was detected in a small lake during the winter. This source was calculated to be only 1% of the quantity of N falling directly on the surface in precipitation. Nitrogen fixation was not detected in a highly eutrophic pond, nor was it detected in a large mainstream reservoir. In both heterocystis blue-green algae and nutrient depletion were never observed. Biological N fixation does not appear to be important in these waters. Transformation of nitrogen was studied in the small lake where nitrogen fixation was detected. A Ceratophyllum-periphyton community is large in biomass and dominates the flora. This community was found to be capable of continuous NH4 assimilation, but could assimilate NO3 most rapidly only in the light. An effort was made to learn if NH4 and NO3 assimilation by this community could be predicted from a knowledge of concentration. Uptake was linear with concentration but the effect of N starvation on the slope of the curve has not been explored as yet. (Author)