The effects of chronic diazepam (D) treatment and exercise training on total body mass (TBM), microsomal protein yield (MPY), calcium uptake by fragmented sarcoplasmic reticulum (SR), muscle fibre cross-sectional area, and both PFK and SDH activities were investigated in the tibialis anterior (TA), soleus (Sol), and plantaris (Plt) muscles of 50 male albino Sprague-Dawley rats. Rats were assigned randomly to control (C), sprint-trained (S), or endurance-trained (E) groups. Training was of 12 weeks duration. One-half of each group received daily intraperitoneally D doses of 5 mg kg-1 of TBM. Exercise reduced TBM (p < 0.05); increased the relative BM of the TA (E = 2.02 +/- 0.02, p < 0.01) and Plt (E = 1.15 +/- 0.02, p < 0.01; S = 1.13 +/- 0.03, p < 0.01), as well as the Ca++ uptake of the Sol SR (C = 0.08 +/- 0.02, E = 0.16 +/- 01, p < 0.05). MPY was elevated in S-Sol (C = 1.12 +/- 0.6, S = 1.52 +/- 0.1, p < 0.01). D elevated Sol MPY as well as TA PFK. S-trained animals had lower mean fibre areas than the E-trained (D-treated and untreated) animals. The elevated relative masses of TA and Plt are explained by a decreased TBM with exercise. The increased Ca++ uptake of the Sol indicates that E enhances this function, and the increased MPY probably implies an increased SR. The D could be responsible for the D-elevated Sol MPY as well as the TA PFK. El D did not reduce neuromuscular activity to a level adversely affecting oxidative enzyme activity, but in the case of PFK activity in the TA muscle, such a reduction was evident.