It was shown that nitrate reductase activity varied between species of Agrostis. Lower values were found in A. setacea and A. stolonifera and higher but variable values occurred in A. tenuis and A. canina. The hypothesis that species or ecotypes within species might have nitrate reductase activities related to soil nitrate or applied fertilizers was examined, but no clear relationship was found. Consideration is given to the difficulties inherent in establishing such correlations. As in current published evidence the relationship between dry matter production and nitrate reductase was often found to be negative rather than positive as might have been expected.