When the mode of contraction of the cat papillary muscle is changed abruptly from isotonic to isometric, the tension of the first isometric contraction is as much as 22% greater and lasts substantially longer than the subsequent stable isometric contractions attained after a few beats. This previously undescribed phenomenon is largely independent of preload or inotropic influences, but is greatly diminished at lower temperatures. Force-velocity curves equivalent to the first isometric contraction revealed a maximum velocity of shortening 9.5 ± 2.0% greater than that of the stable isometric contraction. Thus apparent changes in muscle contractility can occur whenever there are sudden substantial changes in tension development. This effect may be due to transitory changes in free intracellular calcium or, alternatively, to the presence of a viscous element in close association with the contractile element.