The enzymatic hydrolysis of cyclic nucleotide monophosphate in barley seedlings (Hordeum vulgare L., cucltivar Himalaya) was investigated. Multiple forms of enzyme activity toward adenosine-2', 3'-cyclic monophosphate are observed. At least one of these is capable of catalyzing the hydrolysis of adenosine-3', 5'-cyclic monophosphate. Adenosine is the sole product from the hydrolysis of cyclic AMP because of the presence of nucleotidase activity. Further studies on substrate specificity, sedimentation behavior, pH optimum, metal ion effect, dithiothreitol effect and Km valued suggest that the hydrolysis of cyclic nucleoside monophosphate may be due to a nonspecific acid phosphatase.