Measurements of O(2) uptake were made on leaves and roots of two populations of Lolium perenne L. cv S23 (GL66 and GL72), previously shown to have contrasting rates of CO(2) evolution and yields of dry matter. O(2) uptake was faster in the mature leaves of GL66 than those of GL72, but no difference was observed in the respiratory rates of meristematic leaf bases or mature roots. The growth rate of GL72 was faster than that of GL66. Cyanide resistance was substantial in mature leaves but the alternative path did not contribute to O(2) uptake in the dark. In both populations, adding malate and glycine stimulated O(2) uptake, but exogenous sucrose only stimulated when uncoupler was also present. The difference between the respiratory rates of the two populations was maintained under all investigated conditions. We conclude that the rate of mature leaf respiration in the dark in L. perenne is limited by adenylate control of glycolysis. The difference between the fast (GL66) and slow (GL72) respiring populations reflected a greater respiratory capacity and higher turnover of ATP in GL66. Alternative path capacity was also high in the roots of both and contributed substantially to O(2) uptake, as indicated by inhibition by salicylhydroxamic acid in the absence of KCN. The alternative path capacity of meristematic leaf bases was considerably less than that in mature leaves.Transverse and cross-sections were made of mature leaves of both populations to study anatomical features which might explain the differences in ATP turnover, suggested by the biochemical experiments. Leaves of GL72 were thicker but did not show a different anatomy when compared with GL66. The increased thickness was not due to more or larger cells but entirely to a larger intercellular volume.