¹ From the Laboratory of Neurophysiology, National Institute of Mental Health, Bethesa, Maryland 20014, the Department of Biochemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, and the Department of Radiation Biology and Biophysics, University of Rochester Medical Center, Rochester, New York 14620

Electron microscopy shows that the transverse tubular system of frog sartorius swells in Ringer fluid in which NaCl is partially replaced by sucrose (sucrose isotonic solutions). At constant tonicity, the degree of swelling is roughly proportional to the decrease in ionic strength and to the sucrose concentration of the bathing solution. Swelling is time-dependent and reversible within 2 hr. The late after potential which follows a train of impulses is prolonged with swelling, but not to the extent expected from the model of Adrian and Freygang. This discrepancy remains unexplained, as does the mechanism of swelling of the transverse tubular system, although some suggestions are offered. One is that the transverse tubular system contains fixed charges and swells like a fixed charge gel.

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