Morphine is a very potent analgesic which is widely used for treatment of both acute and chronic pain. This drug is highly addictive and its repeated use may cause dependence. Cellular effects of morphine are mediated by opioid receptors (ORs) and their cognate Gi proteins. Since all the basic components of opioid signaling system are present in heart tissue, it is conceivable that morphine may influence function of this organ. Interestingly, it has been observed that morphine can significantly limit tissue damage that occurs during myocardial ischemia (Schultz et al., 1996), but the molecular mechanisms by which morphine acts on the heart are still very poorly understood. The aim of this study was to investigate the effects of chronic (10- and 27-day) administration of low (1 mg/kg/day, i.m.) and high (10 mg/kg/day, i.m.) doses of morphine to rats (male, 290-370 g, n=10 in each group) on the expression of selected G-protein coupled receptors (GPCRs), G-protein subunits and the expression and activity of adenylyl cyclase (AC) in the heart. Whereas 27-day morphine treatment decreased the expression of κ-ORs by about 18 % (Student t-test), 10-day treatment had no effect. Determination of the expression of β-adrenergic receptors by saturation binding studies showed an appreciable increase by 57 % in the total number of these receptors after 27-day treatment with high doses of the drug. Morphine did not affect the expression of any of the selected G-protein subunits (Gsα, Gi/oα, Gzα, Gq/11α and Gβ), but high doses of the drug significantly (by 50-80 %) elevated the amount of AC isoforms type V/VI. Subsequently, the expression level of AC was reduced in proportion to the time after discontinuation of morphine administration. Low doses of morphine increased by 23 % the expression of AC type V/VI only after 27-day administration. These changes were accompanied by increase in AC activity stimulated by Mn2+ (66 %), NaF (91 %), forskolin (73 %) and isoprenaline (105 %). On the other hand, the ability of OR agonists DADLE and U-50488 to inhibit forskolin-stimulated AC activity was rather decreased. Our results indicate that chronic morphine treatment does not primarily induce changes in the amount of myocardial κ-ORs and G-proteins, but significantly enhances the expression of AC V/VI. Increased amount of AC may at least partially explain the observed supersensitization of the enzyme. It can be concluded that prolonged morphine exposure may strongly affect myocardial β-adrenergic receptor-mediated AC signaling.