Field and laboratory experiments measured photosynthetic and growth responses to increased nitrogen of four annual grasses (Avena fatua, Bromus diandrus, B. mollis, Lolium multiflorum). All species had significant correlations between leaf nitrogen and photosynthesis, and all increased similarly in photosynthesis per unit increase of leaf nitrogen. B. mollis, however, had a lower photosynthetic rate than the other species over the range of leaf nitrogen encountered. Root/shoot allocation of biomass and nitrogen were similar for all species. Relative growth rates (RGR) of L. multiflorum and B. mollis were significantly greater than those of A. fatua and B. diandrus. High RGR was related to high unit leaf rate and high leaf,nitrogen content. High nitrogen assimilation rates per unit root weight of L. multiflorum and B. mollis accounted for the greater tissue nitrogen content. L. multiflorum and B. mollis may coexist in annual grasslands with the larger A. fatua and B. diandrus by virtue of their greater nitrogen assimilation rates and higher relative growth rates.