1. The effect of hyperosmolarity on resting tension and on the fluxes of Na and Ca has been characterized in isolated soleus muscles of the rat. 2. When the osmolarity of the incubation medium was increased by the addition of non-permeant solutes (100-400 m-osmole), the tension showed a rapid dos-dependent rise which could be maintained for up to 60 min. 3. Tension development was unaffected by tubocurarine (2 X 10(-5) M), considerably diminished by the omission of Na or Ca from the incubation medium, and inhibited by tetracaine (10(-4) M). 4. The addition of mannitol or sucrose (200 mM) induced a prompt stimulation of the influx of 22Na and 45Ca. Both in the absence and the presence of extracellular Ca hyperosmolarity stimulated the washout of 45Ca from preloaded muscles. Tetracaine (5 X 10(-4 M) suppressed the effects of hyperosmolarity on both the influx and the efflux of 45Ca, but only gave a modest reduction in the stimulation of 22Na influx. 5. Insulin (5-100 mu./ml.) induced a considerable further rise in the resting tension of muscles exposed to mannitol or sucrose (200 mM). This effect was seen in a glucose-free medium and could be abolished by the addition of insulin antibody. 6. It is concluded that hyperosmolarity leads to a rise in the concentration of free Ca2+ ions in the sarcoplasm, partly due to a mobilization of Ca from intracellular pools, but to a considerable extent supplemented from extracelluar sources. Under these conditions, insulin further augments the Ca2+ ion level in the cytoplasm.