Glycerinated muscle fibres labelled with a set of five-membered maleimide spin labels revealed a complex type epr spectra, which showed a reduced rate of anisotropic motion. The spectra can be interpreted in terms of the ratio of the two down-field peaks and of the hyperfine coupling constant. The spectra clearly indicated that the preferred orientation of the 2peta molecular orbital of the odd electron is perpendicular rather than parallel to the long axis of the muscle fibres. Increasing chain length between the meleimide ring and pyrrolidine nitroxide ring reduces the orientational anisotropy. Thermal denaturation destroys the orientation dependence of the epr spectra of spin labels. The Mg-ATP induced shortening of the fibres influences the spectral parameters. The fibers which had been contracted showed a modified orientation dependence of epr spectra.