1. 1. A cyclic 3′,5′-nucleotide phosphodiesterase has been partialy purified from bovine brain cortices. The “purified” enzyme showed little activity unless preincubated with snake venom. Phosphodiesterase as extracted from the cortices was maximally active but gradually lost activity as purification proceeded. Activation by venom was seen only after phosphodiesterase had been purified and was not due to protection by proteins in venom. 2. 2. The venom presumably contained a stimulatory factor(s), which activated the purified but not the crude enzyme. Since 5′-adenosine monophosphate did not affect phosphodiesterase activity, stimulation was not due to relief of product inhibition by 5′-nucleotidase (EC 3.1.3.5) of venom. Based on stability studies, the stimulatory factor could be distinguished from 5′-nucleotidase of venom. 3. 3. The stimulatory factor was labile to extreme pH's and elevated temperatures, but part of the activity survived these treatments. It was not dialyzable and appeared to be a fprotein. Although venom exhibited tryptic activity, neither its tryptic nor stimulatory activity was affected by trypsin inhibitor. Activation of phosphodiesterase by venom appeared to be monomolecular reaction and was not dependent on Mn 2+ at a concentration optimal for phosphodiesterase activity.