The effects of exposing rat soleus muscle to solutions made hypertonic by sucrose, mannitol, or urea for a period of 60 min and then to normal solutions for a further 30 min (hypertonic treatment) on twitch, tetanus, potassium, and caffeine contractures were examined. Following hypertonic treatments, twitch, tetanic, and potassium contracture tension were altered from control. The exact effects were dependent upon the concentration of solute used and the temperature. Electrically evoked contractions were abolished following hypertonic treatment with 400 mM sucrose or mannitol at 37 degrees C but 800 mM at 22 degrees C. Urea was less effective; only after hypertonic treatment with 800 mM were contractions of rat soleus muscle abolished at 37 degrees C, while at the lower temperature contractions could not be abolished. In general, caffeine contractures were reduced by hypertonic treatments at 37 degrees C but not at 22 degrees C. Hypertonic solutions themselves caused contractures of rat skeletal muscle, the amplitude of which directly depended on the concentration of solute used and the temperature. These contractures appeared to be due to the release of intracellular calcium from its stores. In hypertonic sucrose and mannitol but not urea solutions, the twitch was reduced and then abolished. Changes in contractile responses of rat soleus muscle during and after exposure to hypertonic solutions are suggested to be due to osmotically induced changes.