To examine the effects of growth hormone (GH) on the preferential atrophy of the soleus muscle (SOL) occurring after hindlimb suspension (HS), two groups of male rats received daily injections of 2 IU.kg-1 body mass of recombinant human growth hormone (rhGH). Rats were either suspended by the tail for 21 days (HS-GH, n = 5) or nonsuspended (C-GH, n = 5). The effects of rhGH treatment on SOL and extensor digitorum longus muscles (EDL) were compared in two groups of animals receiving daily injections of saline, either suspended by the tail (HS-SA, n = 5) or nonsuspended (C-SA, n = 5). The results showed that the SOL hypertrophy in response to rhGH administration was mostly observed in C rats (+33%, P < 0.01). This increase in muscle mass was correlated with a concomitant increase in the size of type I fibres (+21%, P < 0.05). Although SOL mass decreased during HS in rhGH treated animals (-44%, P < 0.001), the mean normalized mass of this muscle did not significantly differ between C-SA and HS-GH groups. A statistically significant increase in the absolute mass of EDL occurred with rhGH treatment in C-GH (+12%, P < 0.05). The HS-induced decrease in the percentage distribution of type I fibres in SOL was unaffected by the rhGH treatment. In addition, a decrease in the citrate synthase activity in the whole SOL was observed in the two groups of tail-suspended rats (-31%, P < 0.05; -21%, P < 0.05 in SA and GH animals, respectively).2+ f1p4