Following spinal cord injury, muscles below the level of injury develop variable degrees of disuse atrophy. The present study assessed the physiologic changes of the expiratory muscles in a cat model of spinal cord injury. Muscle fiber typing, cross-sectional area, muscle weight and changes in pressure generating capacity were assessed in 5 cats spinalized at the T6 level. Airway pressure (P) generating capacity was monitored during lower thoracic spinal cord stimulation before and 6 months following spinalization. These parameters were also assessed in 5 acute animals, which served as controls. In spinalized animals, P fell from 41 ± l to 28 ± 3 cmH₂O (SE) (p 0.001). Muscle weight of the external oblique (EO), internal oblique (IO), transversus abdominis (TA) and internal intercostal (II) muscles decreased significantly (p 0.05 for each). Muscle weight of the EO, IO, TA and II correlated linearly with P (r 0.7 for each, p 0.05 for each), but not rectus abdominis (RA). Mean muscle fiber cross-sectional area of these muscles was significantly smaller (p 0.05 for each; except RA) and also correlated linearly with P (r 0.55 for each, p 0.05 for each; except RA). In spinalized animals, the expiratory muscles demonstrated a significant increase in the population of fast muscle fibers. These results indicate that following spinalization: a) the expiratory muscles undergo significant atrophy and fiber type transformation and b) the pressure generating capacity of the expiratory muscles falls significantly secondary to reductions in muscle mass.