The calcium ion is now generally considered to be the final activator of the contractile system under physiological conditions. However, it is only recently that this idea has become widely accepted. The physiological role of Ca ion had been almost completely overlooked in the biochemical approach to the study of muscular contraction. Although certain physiologists had demonstrated the contraction inducing effect of Ca ion during this period, their finding did not receive immediate attention. It is rather ironic that recognition of the essential role of Ca ion in contraction has resulted mainly from the investigation into the mechanism of relaxation.

Various reviews have previously been published with regard to the" relaxing factor"(eg Hasselbach, 1964a; Ebashi, 1965b; Smith, 1966; A. Weber, 1966). Therefore, we shall confine the present review concerning the relaxing factor to a brief description of historical aspects and to a presentation of the particular problems which are still under consideration. Emphasis will be placed rather on current views concerning physiological events of muscle based on the" relaxing factor" as well as Ca ion, and also on the molecular mechanism of the action of Ca ion as a regulating agent of the contractile system under physiological conditions. t