To test the hypothesis that an anabolic steroid such as nandrolone decanoate (ND) will ameliorate or abolish disuse-mediated alterations resulting from five weeks of hindlimb immobilization, female Sprague-Dawley rats were divided into six groups: control, control + ND, shortened-immobilized, shortened-immobilized + ND, stretched-immobilized, and stretched-immobilized + ND. Immobilization was accomplished by wrapping hindlimbs with plaster of Paris. Nandrolone decanoate in sesame oil was administered via weekly intraperitoneal injection (7 mg/kg) while nontreated rats received equivalent volumes of the vehicle. After five weeks selected morphometric, biochemical, and mechanical parameters were examined in the slow-twitch soleus muscle (SOL). Muscle wet weight fell from a control value of 145 +/- 19 mg to 70 +/- 8 mg (p less than .05) in the immobilized-shortened group where ND had no effect (81 +/- 11 mg). Muscle stretch alone prevented weight loss (159 +/- 31 mg). The addition of ND resulted in significant SOL hypertrophy (200 +/- 42 mg), p less than .05. Immobilization resulted in a significant shift in muscle protein distribution toward sarcoplasmic protein, a change unaltered by ND but abolished by stretch. Muscle strength, as indicated by peak tetanic tension, fell 45% (p less than .05) as a result of shortened immobilization. Nandrolone decanoate had no effect on this condition, although the ameliorating effect of stretch was abolished when in combination with ND treatment. The elevation observed in maximal shortening velocity (shortened-immobilized) was unaffected by ND alone, but abolished by stretch with or without ND treatment.(ABSTRACT TRUNCATED AT 250 WORDS)