The rate of oxygen uptake was measured on sections of mature leaf tissue derived from two populations ofLolium perenneL. cv. S23, selected for contrasting rates of mature leaf respiration. Under conditions of sufficient nutrient supply, GL72 had a lower rate of respiration and a greater yield than GL66. The populations were exposed to important management variables to investigate their effect on the selection criterion, the two variables being nutrient supply and defoliation. Conditions of low nutrient supply were characterized by a similarity in respiration rate and a change in the regulation of respiration. Enzyme levels, rather than adenylates, controlled respiration. Weekly cutting induced activity of the otherwise latent alternative pathway.