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    Normal, diabetic, and sympathectomized rats were killed at intervals during a 12 h light (6 a.m. to 6 p.m.)/12 h dark cycle to determine (1) the patterns of diurnal glycogen rhythms in skeletal muscle fibre types, and (2) if the absence of insulin or the catecholamines epinephrine and norepinephrine abolished these rhythms. 2. Normal rat muscles lost and restored 40–60% of their glycogen stores during one 24-h cycle. Glycogen losses were linear between 6.30 a.m. (acrophase time for all muscles) and 8.30 a.m. and between 8.30 a.m. and nadir points (4.30 p.m. to 8.30 p.m.), but the rates of loss were non-uniform between these time frames and among fibre types. All muscles had similar linear restoration rates. 3. Glycogen stores in most muscles from diabetic and sympathectomized rats exhibited changes similar to those seen in normal animals. 4. Skeletal muscle fibre types have notable and non-uniform diurnal glycogen rhythms that should be considered in the design of metabolic and exercise experiments. These rhythms do not appear to be regulated, either differentially or in toto, either by insulin or by the catecholamines epinephrine and norepinephrine.

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