Immediate post mortem samples of sternomandibularis muscles from six steers were maintained with a minimum of intrinsic activity at approximately 40 degrees C or allowed to cool to approximately 22 degrees C. Samples were frozen in liquid nitrogen at 0, 2, 4 and 6 h post mortem and serial transverse sections were stained by the periodic acid-Schiff (PAS) reaction for glycogen or reacted for adenosine triphosphatase (ATPase) or succinate dehydrogenase. Individual fibres were mapped and categorized from their ATPase and succinate dehydrogenase activity using a projecting microscope. The absorbance of PAS-stained glycogen in individual fibres was measured with a microscope photometer at 570 and 601 nm with a correction for distributional error. Overall, the post mortem decline in absorbance was approximately twice as fast in body temperature samples relative to room temperature samples. Transient post mortem increases in absorbance were detected in some situations, particularly in fibres with strong ATPase activity from room temperature samples. In fibres with strong ATPase activity, the rate of decline in absorbance increased progressively post mortem. Fibres with weak ATPase mostly had a lower initial post mortem absorbance and were generally the first to become PAS-negative.