Type 1 protein phosphatase (PP1) is a negative regulator of cardiac function. However, studies on the status and regulation of sarcoplasmic reticulum (SR)-associated PP1 activity in failing hearts are limited. We studied PP1 activity and protein and mRNA expression of the catalytic subunit of PP1 (PP1C) and protein levels of PP1-specific inhibitors [inhibitor 1 (Inh-1) and inhibitor 2 (Inh-2)] in the left ventricular (LV) myocardium of 6 dogs with heart failure (HF; LV ejection fraction, 23 ± 2%) and 6 normal dogs. In failing LV tissue, PP1 activity values (expressed as pmol ³²P · min^–1 · mg of noncollagen protein^–1) in the homogenate, crude membranes, cytosol, and purified SR were increased by 52, 54, 55, and 72%, respectively. Trypsin treatment released PP1 but not type 2A protein phosphatase from the SR. In the supernatant of trypsin-treated SR, PP1 activity was ∼24% higher in failing hearts than in normal control hearts. A similar increase in protein expression of PP1C was observed in the nontrypsinized SR. Heat-denatured phosphorylated SR inhibited PP1 activity by 30%, which suggests the presence of Inh-1 or -2 or both in the SR. With the use of a specific antibody, both Inh-1 and -2 proteins were found in the SR; the former was decreased by 56% in the failing SR, whereas the latter did not change. These results suggest that protein phosphatase activity bound to the SR is increased and is predominantly type 1. Increased SR-associated PP1 activity in failing hearts appears to be due partly to increased expression of PP1C and partly to reduced levels of Inh-1 but not Inh-2 protein. Thus inhibition of PP1 activity in the SR appears to be a potential therapeutic target for improving LV function in failing hearts, because it may lead to increased SR Ca²⁺ uptake, which is impaired in failing hearts.