The protein phosphatase inhibitor‐1 (PPI‐1) inhibits phosphatase type‐1 (PP1) only when phosphorylated by protein kinase A and could play a pivotal role in the phosphorylation/dephosphorylation balance. Rat cardiac PPI‐1 was cloned by reverse transcriptase‐polymerase chain reaction, expressed in Eschericia coli, evaluated in phosphatase assays, and used to generate an antiserum. An adenovirus was constructed encoding PPI‐1 and green fluorescent protein (GFP) under separate cytomegalovirus promotors (AdPPI‐1/GFP). A GFP‐only virus (AdGFP) served as control. Engineered heart tissue (EHT) from neonatal rat cardiomyocytes and adult rat cardiac myocytes (ARCMs) were used as model systems. PPI‐1 expression was determined in human ventricular samples by Northern blots. Compared with AdGFP, AdPPI‐1/GFP‐infected neonatal rat cardiomyocytes displayed a 73% reduction in PP1 activity. EHTs infected with AdPPI‐1/GFP exhibited a fivefold increase in isoprenaline sensitivity. AdPPI‐1/GFP‐infected ARCMs displayed enhanced cell shortening as well as enhanced phospholamban phosphorylation when stimulated with 1 nM isoprenaline. PPI‐1 mRNA levels were reduced by 57±12% in failing hearts with dilated and ischemic cardiomyopathy (n=8 each) compared with nonfailing hearts (n=8). In summary, increased PPI‐1 expression enhances myocyte sensitivity to isoprenaline, indicating that PPI‐1 acts as an amplifier in β‐adrenergic signaling. Decreased PPI‐1 in failing human hearts could participate in desensitization of the cAMP pathway.