The aim of the present study was to examine the rate of glycogen mobilization during exercise and the rate of the postexercise glycogen replenishment in different muscle types [white (WG), and red (RG) gastrocnemius, soleus (S) and diaphragm (D)] in rats treated with triiodothyronine (T3, group T). Rats of the control group (C) were treated with saline. The animals were made to run on a treadmill set at 0 degree gradient and at a speed of 1200 m.h-1. The time taken to reach exhaustion in group C was 188 (SD 23) min, whereas in group T, it was only 63 (SD 12) min. The content of glycogen in all muscles of the rats from group T at rest and during exercise was significantly lower than in group C at each corresponding time. At exhaustion, the glycogen content was in WG(C) 34.79 (SD 4.65), (T) 20.10 (SD 4.10); in RG(C) 22.82 (SD 4.66), (T) 16.50 (SD 2.00); in S(C) 14.85 (SD 2.48), (T) 11.90 (SD 2.93); in D(C) 18.18 (SD 3.49), (T) 7.54 (SD 3.36) (mumol of glucosyl units. g-1). The amount of glycogen mobilized during exhausting exercise in RG, S and D was similar in both groups whereas in WG it was much higher in rats of group T than in group C. The concentration of glycogen returned to pre-exercise values in each muscle 3 h after exercise. The net amount of glycogen resynthesized during 3 h of recovery depended on the muscle type. It was in WG(C) 3.30, (T) 18.03; in RG(C) 21.34, (T) 25.88, in S(C) 34.00, (T) 17.68, and in D(C) 17.25, (T) 12.22 mumol of glucosyl units . g-1 (each number represents the difference between the means). It concluded that treatment with T3 markedly affects this exercise-induced metabolism of glycogen in each muscle type. From our study it is suggested that low muscle glycogen content may contribute to a reduction in exercise performance in hyperthyroidism.