AbstractThe oxygen consumption rate (o.c.r.) of pre‐rigor beef muscle has been studied in relation to temperature and to the rates of ATP‐turnover (a.t.r.) and of mitochondrial oxidation (m.r.). The o.c.r. of strips of muscle has a constant enthalpy of activation (ΔH*) of 10–11 kcal/mol (Q10°C = 1.8) over the range of 2–38 °C. From 15–38 °C, the rate‐determining reaction is the resting turnover of ATP (a.t.r.) due to slow, undefined ATP‐hydrolases, which have an identical ΔH* value of about 10.5 kcal/mol. Below 11 °C, the phenomenon of cold‐shortening sets in, due to partial activation of the “contractile” myofibrillar ATPase as the temperature is lowered, with the result that the a.t.r. passes through a minimum at 5 °C, and at 1 °C has become as high as at 15 °C. Under these conditions the a.t.r. is too high for mitochondrial respiration to cope with the ADP produced, so that the m.r. increasingly becomes rate‐limiting.The o.c.r. and a.t.r. of pre‐rigor muscle are both increased by damage, strips showing rates about 2 times and minces about 4 times those given by intact tissue.The r.q. of beef muscle is 1, the substrate almost certainly being the pyruvate derived from glycolysis.The P:O ratio, derived from the observed a.t.r. and o.c.r., has a mean value of 2.8 in beef muscle and of about 2.3 in the muscles of the rabbit, sheep and pigeon. The latter is significantly lower th