There is wide variation in muscle TCr accumulation following supplementation, which can be attenuated by ingestion of a large amount of simple sugars (Green et al, 1996). We demonstrated that muscle glycogen concentration also increases with Cr and carbohydrate (CHO) supplementation and that post-exercise glycogen supercompensation can be enhanced by Cr + CHO supplementation in exercising muscle (Robinson et al, 1999). In this study, we investigated whether Cr supplementation alone can influence muscle glycogen synthesis and storage, and whether any elevated glycogen concentrations were sufficient to improve endurance exercise performance. Six healthy male volunteers participated in the study, which had ethical committee approval. Participants reported to the laboratory on 4 occasions to exercise on a cycle ergometer to the point of volitional exhaustion, after 3 days of a controlled diet. After a familiarisation visit, participants cycled to exhaustion after no supplementation, then two weeks later again cycled to exhaustion after 5 days of CHO supplementation, then 2 weeks later again cycled to exhaustion after 5 days of Cr supplementation. During the experimental visits, muscle and blood samples were obtained before, during and after exercise. Resting muscle glycogen concentrations were significantly elevated above those of the normal dietary condition following CHO supplementation but not by Cr supplementation. At the time of exhaustion of the normal diet without supplementation (TEXnormal), muscle glycogen concentrations were not significantly different at the same time point after CHO or Cr supplementation. Glycogen concentrations at the time of exhaustion after CHO or Cr supplementation (EXHCHO and EXHCr respectively) were also not significantly different from those at TEXnormal. No significant improvements in exercise duration compared with Normal were observed following either CHO or Cr supplementation (Mean [+ SEM]; Normal 92 [10]; CHO 95 [18]; Cr 102 [14] min, p > 0.05). The principal finding of this investigation is that 5 days of Cr supplementation and a normal, controlled diet had no effect on muscle glycogen storage. It would appear that Cr supplementation alone is not sufficient to increase muscle glycogen concentration and that Cr-associated increases in muscle glycogen are as a result of an interaction between the supplementation and other mediators of muscle glucose transport, such as glucose-stimulated insulin release, muscle contraction, or both.