The adaptive response of skeletal muscle to training in normoxia and in severe normobaric hypoxia was studied. The first group of five male subjects trained for 3 weeks on a bicycle (2 h/day, 6 days/week) in normoxia (Control training, Con T). A second group of five subjects trained in an ambient FIO2 decreasing progressively from 12.7% to a final level of 10.0% (hypoxic training, Hyp T). Fourteen months later, these subjects trained in normoxia at the same absolute power (normoxic training, Nor T). Peak oxygen consumption (VO2 max) was measured in normoxic and hypoxic conditions. Biopsies from the vastus lateralis muscle were analysed for fibre size, capillary and ultrastructural composition. Nor T had no effect on muscle tissue or VO2 max. Con T increased volume density of total mitochondria and lipids by 36 and 135% respectively (P < 0.05). Hyp T induced a 10% increase (P < 0.05) in peak VO2 max measured in hypoxia. Mean fibre cross-sectional area, interfibrillar mitochondrial volume density and capillary-to-fibre ratio were increased (P < 0.05) by 10, 42 and 13% respectively in the Hyp T group. These results suggest that training at the same relative workload in normoxia and hypoxia have similar, but not identical, effects on muscle tissue. If training in normoxia is carried out at the same absolute workload as in severe hypoxia, no significant effects are observed.