The possible alteration of metabolism of the m. vastus medialis was investigated during exercise after eccentric loading. Twelve male subjects performed stepping exercise for 25-60 min. One week before and 24 h after stepping they performed concentric leg exercise at stepwise increasing intensity, while power output was measured. During this concentric exercise and recovery therefrom, phosphorous metabolites were also measured in the m. vastus medialis with magnetic resonance spectroscopy. Creatine kinase and myoglobin in blood were measured before and 72 h after stepping. T1 and T2 1H-relaxation times for water were calculated from magnetic resonance images collected 72 h after stepping, and used as measures for oedema. The subjects perceived substantial soreness from 24 to 72 h after stepping. The ratio of inorganic phosphate over phosphocreatine at rest increased from 0.12 +/- 0.02 (before) to 0.19 +/- 0.04 (24 h after stepping) (P < 0.05). Creatine kinase activity was slightly elevated 72 h after stepping (71 [49-812] U L-1 [median, range]; P < 0.05) compared with baseline values (58 [26-409] U L-1), whereas myoglobin concentration was not significantly elevated (15 [8-120] micrograms L-1 compared with 8 [8-41] micrograms L-1). In the eccentrically exercised muscles, T1 and T2 values were not or only slightly higher than in the concentrically exercised contralateral muscles. The relation between power and the ratio of inorganic phosphate over phosphocreatine during concentric exercise, and the recovery data for inorganic phosphate, phosphocreatine and pH did not alter after stepping. These data suggest that quadriceps metabolism during concentric exercise and recovery therefrom is not affected by prior eccentric overload, but it cannot be excluded that metabolism will alter during exercise after more strenuous prior eccentric overload.