134 Recently, glucose transporter 4 (GLUT4) knockout (KO) mice have been generated (Katz, E.B. et al. Nature 377:151-155, 1995). KO mice have normal glucose tolerance, whereas insulin tolerance is impaired, suggesting insulin resistance in muscle. Here we examined the effect of GLUT4 ablation on post-exercise glucose uptake and glycogenesis. Fasted KO and wild-type (WT) mice underwent a 3 hr swim bout (EX) and were then assigned to 0, 5, or 24 hr post-exercise groups with free access to chow and water. EX and insulin independently induced a 2.1-fold increase (p<0.05) in[3H]-2-deoxyglucose uptake in extensor digitorum longus muscle from WT, with no effect of either stimuli noted in KO. Fed gastrocnemius muscle glycogen was similar in WT vs. KO mice (21±1 vs. 19±1 mmol/kg, respectively), and was reduced 65% (p<0.05) in WT and KO following EX. WT glycogen content increased to 34±3 mmol/kg (vs. fed, p<0.05) 5 hr post-EX, with no difference at 24 hr. KO muscle glycogen remained depleted 5 hr post-EX, but was completely restored 24 hr post-EX (21±2 mmol/kg vs. EX, p<0.05). Fed liver glycogen was higher in KO vs. WT (310±31 vs. 136±26 mmol/kg respectively, p<0.05) and was reduced by >90% by EX (p<0.05). 5 hr post-EX WT liver glycogen was elevated to a maximum level of 414±18 mmol/kg (vs. fed, p<0.05). In KO mice, liver glycogen was increased to 389±24 mmol/kg 5 hr post-EX, (vs. fed, p<0.05) with a further increase to 530±45 mmol/kg 24 hr post-EX (vs. 5 hr post-EX, p<0.05). In conclusion, GLUT4 is essential for exercise induced glucose transport and muscle glycogen super-compensation, but not for muscle glycogen repletion. Furthermore, liver glycogen content is greater in KO than WT.