The rate constant for dissociation of ADP from actomyosin subfragment 1 (S1) has been measured in this laboratory and elsewhere for a variety of vertebrate muscle types. We have made the following observations: (i) In solution, the dissociation of ADP from actomyosin-S1 limits the rate of dissociation of actomyosin-S1-ADP by ATP and, presumably, also limits the rate of crossbridge detachment in contracting muscle. (ii) For muscle types in which the rate of ADP dissociation from actomyosin-S1 is slow enough to measure using stopped-flow methods, the rate constants are nearly the same as the theoretical value for the minimum allowable rate constant for dissociation of an attached crossbridge. Therefore, ADP dissociation is sufficiently slow to be the molecular step that limits the maximum shortening velocity of these muscles. (iii) Variation with muscle type of the rate constant for ADP dissociation may be a general phylogenetic mechanism for regulating shortening velocity.