1 Phosphoinositide-specific phospholipase C (PLC) is involved in the regulation of many cellular functions. In the myocardium, PLC-generated second messengers play a role in the regulation of contractile function and in the pathophysiology of myocardial hypertrophy. 2 In the present study, the effect of mastoparan, a tetradecapeptide which is capable of activating heterotrimeric G proteins by mimicking the action of an activated receptor, on membrane-bound human myocardial PLC, was investigated in a cell-free assay with exogenous phospholipids as a substrate. 3 Mastoparan stimulated human myocardial PLC approximately two fold with a half-maximal effect at approximately 2 microM and a maximal effect at 10 microM. The peptide did not alter the dependence of PLC on free calcium ions. In order to exclude non-specific effects of mastoparan due to its amphiphilic properties, different mastoparan derivatives were used as positive and negative controls. Mas17, an inactive mastoparan analogue with physical properties very similar to mastoparan, did not induce substantial PLC stimulation in human myocardial membranes. In contrast, Mas7, the most active mastoparan derivative known, caused a more pronounced PLC activation compared with the mother compound indicating that the effect was sequence-specific. Human myocardial PLC stimulation was pertussis toxin-insensitive and could not be abolished by addition of excess alpha-subunits from purified retinal transducin or by excess GDP or GDP/beta S. In order to investigate whether mastoparan stimulate PLC via pertussis toxin-insensitive alpha q, a deletion mutant of PLC beta 2 deficient of the site of interaction with alpha q-subunits was expressed in COS-1 cells. Both wild-type and mutant PLC beta 2 were similarly sensitive to stimulation by mastoparan. It is concluded that mastoparan stimulates human myocardial PLC by a mechanism distinct from heterotrimeric G proteins.