Autoradiographs consisting of a 1000 A thick tissue section and a 1400 A thick emulsion film have been prepared from frog toe muscles labeled with Ca45. The muscles had been fixed with an oxalate-containing osmium solution at rest at room temperature, at rest at 4°C, during relaxation following K+ depolarization or after prolonged depolarization. From 6 to 39 per cent of K+ contracture tension was produced during fixation. The grains in the autoradiographs were always concentrated in the center 0.2 to 0.3 µ of the I band and the region of the overlapping of the thick and thin filaments. The greater the tension produced during fixation, the greater was the concentration in the A band and the smaller the concentration in the I band. Autoradiographs of two muscles fixed by freeze-substitution resembled those of muscles which produced little tension during osmium fixation. Muscles which shortened during fixation produced fewer grains. In the narrow (<2.0 µ) sarcomeres of the shortened muscles, grain density decreased with decreasing sarcomere width. A theoretical analysis of the significance of these grain distributions is proposed and discussed.

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