1. The contracture normally induced in isolated bovine tracheal smooth muscle by potassium-rich solution was abolished by removal of the extracellular calcium. The contraction returned when calcium was added to the solution in a concentration greater than 0.05 mmol/l. 2. The amplitudes of the potassium-contracture, and of the contractile responses to histamine and acetylcholine in normal physiological solutions, declined at low temperatures (15-25 degrees C). If drugs were added during the plateau phase of the potassium contracture, the extra tension developed above the contracture did not change with temperature. 3. Calcium-depletion reduced the responses to drugs, and repeated application of the drugs in calcium-free solution produced progressively smaller contractions, suggesting that an intracellular store of calcium was being used up. 4. Depolarization of calcium-depleted tracheal muscle by high-K+ solution without calcium produced responses to drugs which were larger than those in sodium-based calcium-free solutions. There was no potentiation in a solution in which sodium was replaced by sucrose, suggesting that potassium was not acting simply by replacing the sodium. 5. It is suggested that depolarization of the membrane by potassium makes available a fraction of bound calcium which was not available in calcium-free sodium-based solution.