When a partially purified dog liver phosphorylase phosphatase preparation was incubated with ATP and Mg ions, its activity increased several fold. A slower activation was observed in the presence of Mg ions only. When the same preparation was preincubated with ATP alone, complete inactivation was obtained. The ATP pretreated enzyme could then be reactivated by Mg++, as well as by ATP and Mg++. The activation and the inactivation of the phosphorylase phosphatase, were shown to be dependent on time, temperature and the concentration of ATP and Mg ions. Neither on the activation nor on the inactivation of the phosphorylase phosphatase could and effect of cyclic 3′ :5′ ‐AMP be observed. Copyright © 1969, Wiley Blackwell. All rights reserved
