In ventricular myocytes, phenylephrine (PE) and endothelin (ET) stimulate phosphoinositide (PI) hydrolysis, cell growth, and expression of several genes [e.g., atrial natriuretic factor (ANF)] often associated with cardiac hypertrophy. In this study the production of inositol monophosphate (InsP) and diglyceride (DG), both products of PI hydrolysis, and ANF were monitored during long-term exposure (up to 72 h) to PE or ET. For PE, InsP production increased and continued unabated throughout the time course; DG levels also increased and remained elevated, and similar rates of ANF production were observed from 24 to 72 h. For ET, the initial InsP and DG responses equaled those to PE, but diminished by 24 h. ET-stimulated ANF production equaled the PE response at 24 h but subsided by 72 h. Adding PE to cells previously desensitized to ET elicited maximal InsP formation, indicating the desensitization was ET specific. These data emphasize that while phenylephrine and endothelin have similar initial effects on cardiac second messenger production and ANF expression, hormonally specific patterns develop over extended periods of exposure.