The effects of in vivo and in vitro hyperosmolality on skeletal muscle performance in the amphibians Rana pipiens and Scaphiopus couchii.

1. The effect of in vivo and in vitro hyperosmolality on skeletal muscle function was investigated in two species of anuarans Scaphiopus couchii and Rana pipiens. 2. Muscle contractile performance, measured as peak tetanic tension declined to greater degree when tissue dehydration occurred in vitro rather than in vivo, even though tissue water contents were greater in vivo. 3. The muscles from S. couchii, a more dehydration tolerant species than R. pipiens, maintained tension at lower tissue water contents than R. pipiens. 4. Data for the effects of in vivo dehydration on plasma sodium, urea and osmotic concentration, as well as tissue water contents, are also presented for both species.