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Stretching of K-depolarized contracted helically cut strips of pig coronary arteries produced a delayed increase in tension. The influence of temperature and extracellular calcium on this active response to stretch was investigated. Reference for all tension values was the amplitude of contraction (induced by K-depolarization) starting from the high resting tension. 1. The mean amplitude of the delayed tension increase after stretch amounted to 27.5 plus or minus 4.8% (x plus or minus SEM, n equals 9); lowering the bath temperature from 37 degrees C to 27 degrees C caused a drop to 10.8 plus or minus 1.5% (n equals 9; P smaller than 0.0025); increasing the calcium concentration of the bath solution from 2.7 to 6.9 mM produced negligible effects on both the amplitude of the delayed tension increase (24.6 plus or minus 1.0%; n equals 9), and the amplitude of contraction after depolarization. 2. The peak tension time of the active response to stretch was not changed by the 6.9 mM calcium, but prolonged from 27.9 plus or minus 4.0 to 40.7 plus or minus 4.4 minutes (P smaller than 0.025) by lowering the bath temperature to 27 degrees C. At the high calcium level the preparation contracted faster after depolarization than at the normal calcium level. 3. The experimental results correspond with the conception of the sliding filament mechanism as the basic process of contraction also in vascular smooth muscle preparations. The delayed tension increase after stretch may be caused by a recruitment of interaction sites between contractile proteins.