Glycerol-extracted rabbit psoas muscle, when bathed in adenylylimidodiphosphate (AMP-PNP), at pH 8·0, gives rise to low-angle X-ray diffraction diagrams which are different from the patterns characteristic of either relaxed or rigor muscle. At 0°C, the rigor cross-bridge pattern is largely abolished, though there is evidence for some rigor-like links. There is an increase in relative intensity of the meridional reflections at 144and 72 . This may indicate that the myosin projections are better ordered according to the repeat of the thick filament backbone or are oriented more perpendicular to the filament axis. Improved lateral register may also contribute to this increase. The ratio of the two major equatorial reflections, I 11 / I 10 , is reduced to a value midway between rigor and relaxed muscle. These effects are very temperature-dependent. At 20°C, the rigor cross-bridge pattern is strong. The change in the ratio I 11 / I 10 at 20°C is within the range of experimental error (10%). However, there is an increase in the relative intensity of the 72meridional, which may, as before, indicate improved order according to the thick filament repeat, or more perpendicular orientation. Thus AMP-PNP changes the actin-myosin bonds (as seen by X-rays), particularly near 0°C, but does not restore the thick filament to the configuration typical of relaxed muscle. It is likely that much of the pattern is due to the A·M·AMP-PNP moiety which is not ordered with respect to the actin helix in the same way as the simple actin-myosin (rigor) complex.