Opioids have long been known for their analgesic effects and are therefore widely used in anesthesia and intensive care medicine. However, in the last decade research has focused on the opioidergic influence on cardiovascular function. This project thus aimed to detect the precise cellular localization of kappa opioid receptors (KOR) in left ventricular cardiomyocytes and to investigate putative changes in KOR and its endogenous ligand precursor peptide prodynorphin (PDYN) in response to heart failure. After IRB approval, heart failure was induced using a modified infrarenal aortocaval fistula (ACF) in male Wistar rats. All rats of the control and ACF group were characterized by their morphometrics and hemodynamics. In addition, the existence and localization as well as adaptive changes of KOR and PDYN were investigated using radioligand binding, double immunofluorescence confocal analysis, RT-PCR and Western blot. Similar to the brain and spinal cord, [3H]U-69593 KOR selective binding sites were detected the left ventricle (LV). KOR colocalized with Cav1.2 of the outer plasma membrane and invaginated T-tubules and intracellular with the ryanodine receptor of the sarcoplasmatic reticulum. Interestingly, KOR could also be detected in mitochondria of rat LV cardiomyocytes. As a consequence of heart failure, KOR and PDYN were up-regulated on the mRNA and protein level in the LV. These findings suggest that the cardiac kappa opioidergic system might modulate rat cardiomyocyte function during heart failure.