Myostatin, a member of the TGF-beta superfamily, is a key negative regulator of skeletal muscle growth. The role of myostatin during skeletal muscle regeneration has not previously been reported. In the present studies, normal Sprague-Dawley and growth hormone (GH)-deficient (dw/dw) rats were administered the myotoxin, notexin, in the right M. biceps femoris on day 0. The dw/dw rats then received either saline or human-N-methionyl GH (200microg/100g body weight/day) during the ensuing regeneration. Normal and dw/dw M. biceps femoris were dissected on days 1, 2, 3, 5, 9 and 13, formalin-fixed, then immunostained for myostatin protein. Immunostaining for myostatin revealed high levels of protein within necrotic fibres and connective tissue of normal and dw/dw damaged muscles. Regenerating myotubes contained no myostatin at the time of fusion (peak fusion on day 5), and only low levels of myostatin were observed during subsequent myotube enlargement. Fibres which survived assault by notexin (survivor fibres) contained moderate to high myostatin immunostaining initially. The levels in both normal and dw/dw rat survivor fibres decreased on days 2-3, then increased on days 9-13. In dw/dw rats, there was no observed effect of GH administration on the levels of myostatin protein in damaged muscle. The low level of myostatin observed in regenerating myotubes in these studies suggests a negative regulatory role for myostatin in muscle regeneration.