Plakoglobin (γ‐catenin), a member of the armadillo family of proteins, is a constituent of the cytoplasmic plaque of cardiac junctions and is involved in anchorage of cytoskeletal filaments to specific cadherins. Its genetic inactivation leads to an embryonic lethal phenotype due to heart dysfunction related to an impairment in the architecture of intercalated discs and in the stability of the heart tissue. To elucidate the functional consequences of the loss of plakoglobin for myofibrillar function, we monitored passive stress–strain relationship and contractility parameters of demembranated embryonic fibers. Heart fibers obtained from plakoglobin‐deficient embryonic mice were significantly less compliant than were fibers from wild‐type embryos. This difference was especially pronounced at lower fiber extension levels: at 120% of slack length, compliance was 2.5‐fold lower in plakoglobin‐deficient mice than in the corresponding wild‐type group. Contractile paramenters (force per cross‐section; Ca²⁺ sensitivity of isometric force and shortening velocity at near‐zero load) were comparable in all experimental groups. Therefore, we suggest that plakoglobin is important for cardiac compliance but not necessary for the attachment of the myofibrillar apparatus to adherens junctions. Thus, we conclude that the loss of function of desmosomes and the profound disarrangement of junctional components in plakoglobin null embryos is associated with a decreased passive compliance, which may explain the ventricular rupture and consequent pericardial tamponade in embryos lacking plakoglobin. J. Cell. Biochem. 72:8–15, 1999. © 1999 Wiley‐Liss, Inc.