1. 1.|The kinetic behavior of pyruvate kinase (PyK) from liver of the Arctic ground squirrel ( Citellus undulatus) was examined over a wide temperature (5–37°C) range in the presence of its substatres and modulators, and cations that are known to influence activity of this enzyme. 2. 2.|The substrates adenosine diphosphate (ADP) and phosphoenolpyruvate (PEP) as well as the modulators fructose 1,6-bisphosphate (FBP) and adenosine triphosphate (ATP) appear to be bound in such a manner that a decrease in the S 0.5 value is accompanied by an increase in the Hill constant ( h), indicating an enhanced degree of site-site interaction in the binding of these metabolites. 3. 3.|In marked contrast, the cations K +, Mg 2+ and Mn 2+, are bound in such a manner that falling S 0.5 values are accompanied by decreases in the attendant h values. 4. 4.|The significance of these results is discussed and it is suggested that only the intermediates play a role in regulating activity of liver pyruvate kinase while the ions appear to enhance the Q 10 effect on the PyK reaction, and function as an integral part of the intracellular milieu that maintains the efficiency of enzyme regulation over a wide temperature range.