AbstractA study has been made of the onset of rigor mortis in the longissimus dorsi muscle of the ox, thus extending similar observations previously made on rabbit muscle. The changes in pH and adenosine triphosphate (ATP) content accompanying the physical manifestations of rigor are described. As in rabbit muscle, the development of inextensibility accompanies the dephosphorylation of ATP, and, provided the ultimate pH is low, the fall in pH runs parallel to these changes. The rate of decrease in pH is almost identical with that in rabbit muscle at physiological temperature, but differs appreciably from it at lower temperatures. The results are discussed in relation to the time required for the completion of the onset of rigor mortis in commercial chillers.