Annexins are characterized by Ca²⁺-dependent binding to phospholipids. Annexin II mainly participates in cell-cell adhesion and signal transduction, whereas annexins V and VI also seem to regulate intracellular calcium cycling. Their abundance and localization were determined in left ventricle (LV) and right ventricle (RV) from hypertensive guinea pigs, during the transition from compensatory hypertrophy to heart failure. Immunoblot analysis of annexins II, V, and VI revealed an increased accumulation (2.6-, 1.45-, and 2.3-fold, respectively) in LV from hypertensive guinea pigs and no modification in RV. Immunofluorescent labeling of annexins II, V, and VI; of Na⁺-K⁺-ATPase; and of sarcomeric α-actinin showed that in control LV and RV,1) annexin II is present in nonmuscle cells; 2) annexins V and VI are mainly observed in the sarcolemma and intercalated disks of myocytes; 3) annexins II, V, and VI strongly label endothelial cells and adventitia of coronary arteries; and 4) annexin VI is present in the media. At the onset of heart failure, the most striking changes are the increased protein accumulation in LV and the very strong labeling of annexins II, V, and VI in interstitial tissue, suggesting a role in fibrosis development and cardiac remodeling.