The effects of 4 wk of detraining on maximal O2 uptake (VO2max) and on endurance capacity defined as the maximal time to exhaustion at 75% of VO2max were studied in nine well-trained endurance athletes. Detraining consisted of one short 35-min high-intensity bout per week as opposed to the normal 6-10 h/wk. Detraining had no effect on VO2max (4.57 +/- 0.10 vs. 4.54 +/- 0.08 l/min), but endurance capacity decreased by 21% from 79 +/- 4 to 62 +/- 4 min (P < 0.001). Endurance exercise respiratory exchange ratio was higher in the detrained than in the trained state (0.91 +/- 0.01 vs. 0.89 +/- 0.01; P < 0.01). Muscle [K+] values were unchanged during exercise and were similar in the trained and detrained states. Muscle [Mg2+] values were similar at rest and at minute 40 (30.3 +/- 0.9 vs. 30.8 +/- 0.6 mmol/kg dry wt) but increased significantly at exhaustion to 33.8 +/- 1.0 mmol/kg dry wt in the trained state and to 33.9 +/- 0.9 mmol/kg dry wt in the detrained state. The elevated muscle [Mg2+] at exhaustion could contribute to fatigue in prolonged exercise through an inhibition of Ca2+ release from sarcoplasmic reticulum. It is concluded that the endurance capacity can vary considerably during detraining without changes in VO2max. Altered substrate utilization or changes in electrolyte regulation may account for the reduced endurance capacity.