Trimmed strips of sternomandibularis muscles taken from freshly-slaughtered cattle were placed in an isotonic myograph and cooled to 1 degree C. Spontaneous activity due to neuromuscular irritability was minimized by keeping muscle surfaces moist and anaerobic and was monitored by electromyography. Muscle strips were removed and frozen for histochemical analysis after they had completed their initial phase of cold-induced shortening (several hours). Control strips maintained for an equal time at 24 degrees C rarely depleted the stainable glycogen in any of their muscle fibres so as to become PAS-negative. In chilled muscle strips, however, glycogenolysis was activated in some muscle fibres and they became PAS-negative. In serial sections, most of the PAS-negative fibres exhibited strong ATPase and weak succinate dehydrogenase activity. Fibres with weak ATPase and strong succinate dehydrogenase activity rarely became PAS-negative. These results are in agreement with biochemical reports of a cold-induced (less than 5 degrees C) activation of glycolysis in skeletal muscle post mortem. Investigations on untrimmed lengths of excised sternomandibularis muscle indicated that longitudinal muscle damage caused in cutting muscle strips for the myograph and/or their more rapid rate of initial cooling had facilitated the depletion of stainable glycogen.