Intracellular concentrations of cyclic nucleotides are regulated  by cyclic nucleotide phosphodiesterases and one form of this enzyme is activated by Ca²⁺ and calmodulin. Calmodulin-dependent cyclic nucleotide phosphodiesterases (PDE 1) have been intensively studied and are the best characterized phosphodiesterases. In this paper, we briefly review some properties of bovine brain PDE1 and the effect of an antiparkinsonian agent, deprenyl (selegeline hydrochloride) – a selective inhibitor of monoamine oxidase-B, on bovine brain PDE1 isozymes. Bovine brain PDE1 exists in two major isozymes and they are designated as PDE1A2 and PDE1B1 isozymes. Our studies indicate that deprenyl primarly inhibits PDE1A2, however the inhibition PDE1B1was observed to a lesser extent. The inhibition of PDE1A2 was overcome by increasing the concentration of calmodulin suggesting that deprenyl may be calmodulin antagonist or may act specifically and reversibly on the action of cadmodulin.  The PDE1A2 is predominantly  expressed in brain and its inhibition can result in increased intracellular levels of cAMP. The increased intracellular levels of cAMP have a protective effect on dopaminergic neurons. Deprenyl may be a valuable tool to investigate the physiological roles of brain PDE1 isozymes in the progression of Parkinson’s  disease and provides a new insight into the action of this drug.