1. Strips of bovine tracheal muscle and rabbit aorta produced sustained contractions on perfusion with Krebs solution made twice normal strength by addition of sucrose. The contractures were relaxed on return to normal Krebs solution. 2. Similar contractures were produced by tracheal muscle strips in Krebs solutions made twice normal strength by addition of galactose, glucose or NaCl whereas urea caused only a transient contraction. 3. In twice normal strength Krebs solution (sucrose added) the basal tension of rat portal vein and guinea-pig taenia coli was increased. Spontaneous mechanical activity was maintained, but the frequency of contractions was reduced. 4. The hypertonic contracture of bovine trachea in twice normal strength Krebs solution (sucrose added) was reduced by 15% by omission of Ca from the bathing fluid (0.1 mmol/l EGTA added). Severe Ca depletion, by prolonged washing in Ca-free Krebs with 12.5 mmol/l EGTA and Carbachol added, resulted in a 77% reduction in the hypertonic contracture. 5. In twice normal Krebs solution (sucrose added), the hypertonic contracture was partially relaxed by isoprenaline (4 x 10(-6) mol/l); the contractile response to carbachol was reduced; the contractile response to high-K Krebs solution was maintained. 6. Atropine (5 x 10(-7) mol/l) abolished the contractile response to carbachol, but had no effect on the hypertonic contracture. 7. It is suggested that the contraction of bovine tracheal strips in hypertonic solutions is mainly due to activation of the contractile myofilaments rather than simple cell shrinkage. Hypertonic solutions may also interfere with some steps in the excitation-contraction coupling sequence.