The purpose of this study was to assess the effect of exercise intensity during an incremental exercise test on plasma Mg concentration in well-trained euhydrated athletes. Twenty-seven well-trained endurance athletes carried out a cycloergometer test: after a warm-up of 10 min at 2.0 W·kg(-1), the workload increased by 0.5 W·kg(-1) every 10 min until exhaustion. Oxygen uptake (VO(2)), blood lactate concentration ([La(-)](b)), catecholamines, and plasma Mg were measured at rest, at the end of each stage and at 3, 5 and 7 minutes post-exercise. Urine specific gravity (U(SG)) was analyzed before and after the test, and subjects drank water ad libitum. Fat oxidation rate (FAT(oxr)), carbohydrate oxidation rate (CHO(oxr)), energy expenditure from fat (EE(FAT)), energy expenditure from carbohydrate (EE(CHO)), and total EE (EE(TOTAL)) were estimated using stoichiometric equations. Plasma Mg concentration at each relative exercise intensity (W·kg(-1)) were compared by means of repeated-measures ANOVA. Pearson's correlations were performed to assess the relationship between variables. The significance level was set at p<0.05. No significant differences were found in U(SG) between before and after the test (1.014±0.004 vs 1.014±0.004 g·cm(-3)). Nor were significant differences found in plasma Mg as a function of the different exercise intensities. Further, no significant correlations were detected between Mg and metabolic variables. In conclusion, acute exercise at a range of submaximal intensities in euhydrated well-trained endurance athletes does not affect plasma Mg concentration, suggesting that the plasma volume plays an important role in Mg homeostasis during exercise.