The fine structural pattern of glycogen storage in resting and sprint-exercised human vastus lateralis muscle fibres of different types was analysed using ultrahistochemical methods. Three male subjects (31-36 years) performed 60 consecutive, supramaximal bouts of bicycle exercise, each starting every 1 min and having a duration of 8 s (including approximately 3 s of acceleration). The load was estimated to correspond to 200% of VO2-max. Five other subjects (22-27 years) constituted controls. Ultrathin sections stained with periodic acid-thiosemicarbazide-silver proteinate (PA-TSC-SP) clearly revealed a compartmental distribution of glycogen. Glycogen is stored at five topographically, and probably also functionally, different locations. They are the subsarcolemmal, intermyofibrillar, para-Z-disc, N2-line, and H-zone spaces. During the exercise, glycogen from the N2-line and para-Z-disc locations is preferentially utilized. Serial sections stained with uranyl acetate and lead citrate demonstrated that glycogen stores of the type 2 fibres were more depleted than those of type 1 fibres. The implications of the differential intracellular glycogen storage are discussed