Intracellular neutral proteinases activity of bovine coronary arteries was demonstrated, isolated and partially characterized. The enriched crude enzyme preparation can be inhibited by diisopropylphosphofluoridate (DFP) and phenylmethylsulfonylfluoride (PMSF). The inhibitor concentration (pI50 values) by DFP and PMSF in the enzyme preparation and for trypsin are very similar. Several drugs (isoproterenol, propranolol, theophylline) tested on the enriched crude neutral proteinases preparation showed stimulatory or inhibitory activities. The results are discussed with regard to a possible action mechanism such proteinases on the cyclic AMP/cyclic GMP system.