We have compared the form and dimensions of the Z-band lattice in rat papillary muscle fixed at rest with and without ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) using electron microscopy and optical diffraction. In unstimulated muscle, the Z-band lattice form called basket weave predominated, and the Z-spacing (defined as the repeat distance of a tetragonal array of cross-cut thin filaments from the same sarcomere) was 23.93 +/- 0.37 nm. Muscles exposed to EGTA exhibited the small square-lattice form, and the Z-spacing was 20.50 +/- 0.19 nm. The Z-spacings in the two lattice forms were similar in cardiac and skeletal muscles such that the decrease in Z-spacing in the transition from basket weave to small square in this study was similar to the increase in Z-spacing previously demonstrated in skeletal muscle in the transition from small square to basket weave. The Z-lattice form and dimensions in unstimulated cardiac muscle resembled those in tetanized skeletal muscle. These findings are consistent with the higher resting tension in cardiac muscle and suggest that Ca2+ may be important for the maintenance of the expanded Z-lattice form.