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Tench (Tinca tinca) were acclimated to either aerated (P02 17.6 KPa) or hypoxic water (P02 1.5 KPa) at 15 degrees C. Fish acclimated to P02 17.6 KPa had a routine oxygen consumption (mls O2/Kg bodyweight/h) of 32.7 in aerated water. Upon acute exposure to Po2 1.5 KPa oxygen consumption decreased to 10.8 and 15.6 in fish acclimated to aerated and hypoxic water, respectively. On the basis of staining for glycogen and for the activities of myofibrillar ATPase and succinic dehydrogenase, three main fibre types can be differentiated in the myotomal muscle. Fibres have been classified as slow, fast aerobic and fast glycolytic. Fast aerobic fibres can be distinguished histochemically by their alkaline-stable Ca2+-activated myofibrillar ATPase activity and their intermediate levels of staining for glycogen and succinic dehydrogenase activity. The patterns of innervation of the fibre types have been investigated by staining neuromuscular endplates and peripheral axons for acetylcholinesterase activity. Motor axons to slow fibres branch extensively giving rise to a number of diffuse endplate formations on the same and adjacent fibres. Fast glycolytic fibres also have a complex pattern of innervation with 8-20 endplates per fibre. A large proportion of the endplates belonged to separate axons. Cross-sectional areas and perimeters of fibres, the number of capillaries/fibre and the lengths of contacts between capillaries and fibres were determined from low-magnification electron micrographs. Acclimation to hypoxia resulted in a decrease in the number of capillaries per fibre for both slow (1.8 to 1.0) and fast (0.8 to 0.2) muscles. The capillary perimeter supplying 1 micrometer 2 of fibre cross-sectional area decreased by 43% and 76% for slow and fast fibres, respectively.