Until recently, rats have been used for studies on the effects of anabolic steroids (AS) for improving physical performance. However, no consistent results on the changes in muscle fibre volume have ever been obtained. In this study we investigated the morphological effects of nandrolone phenylpropionate, an AS, on the muscle fibres of the diaphragm in adult male mice. AS (5 mg kg(-1) week(-1)) was injected intramuscularly in the experimental group (AS-administration group), and peanut oil (0.08 ml week(-1)) was injected in the control group. The cross-sectional areas of Type I (red) and Type II (white and intermediate) muscle fibres after 4 weeks of AS administration were 135% and 139% larger than those in the control group, respectively. The mean cross-sectional areas of mitochondria in the subsarcolemmal (SS) region and interfibrillar (IF) region in Type I fibres and the SS region of the Type II muscle fibres in the AS-administration group were 139%, 135% and 124% larger than those in the control group, respectively. No significant differences in the cross-sectional area of mitochondria were noted between the group of mice administered with AS for 8 weeks and the control group, showing that the effect of the AS administration attained a peak at 4 weeks, but that the effect of the 8 weeks of AS administration declined. We concluded from these results that the response of Type I muscle fibres to AS is stronger than that of Type II muscle fibres, as is the response of SS mitochondria compared to IF mitochondria. We also suggested that the two types of mitochondria may have different roles and that administration of AS may cause a different response in the two types of mitochondria.