Objectives: The aim of this thesis is to demonstrate the potential positive or negative effect of the tannic acid on the cardiac muscle of hyperglycemic rats and the expression of the connexins 43 using a concentration that has already shown beneficial effects. Material and Methods: In this trial twelve male Wistar rats were divided into three groups based on beverages they were consuming. There were four rats in each group and Trial lasted twenty weeks. The control group consumed clean drinking water and the second group consumed 30% aqueous sucrose solution. Third group consumed 30% aqueous solution of sucrose and 0,1% tannic acid. They were weighed on electronic balance and their glucose concentrations in plasma were determined with glucometer at the same time. Animals were euthanized and their hearts were removed. The tissue was prepared and then stained by Mallory's trichrome stain. Tissues were also analysed with immunohistochemistry and then microphotographies were taken. Microphotographies were analyzed by Adobe photoshop 7 and Image J (National Institutes of Health, Bethesda, MD, USA). GraphPad Prism 5 (McIntosh, CA, USA) was used for statistical analysis. Results: After 20 weeks of experiment blood glucose levels of group that had consumed 30% aqueous solution of sucrose significantly increased compared to the control group (p <0,05). Blood glucose levels in the group of rats that additionally consumed tannic acid (0,1%) blood increased less. Also there was significant weight reduction (p <0,05) in that group. After analysis we found that damages of the heart caused by high blood glucose concentration and consumation of tannic acid were significantly different among the groups (p <0.05). Tannic acid did not caused statistically significant (p <0,05) reduction or increase in the expression of connexins 43 in the heart because of great variability in group that consumed tannic acid. Conclusion: We concluded that heart muscle damage is significantly different among the groups. Tanic acid together with hyperglycemia in a dose of 0.1% does not cause a significant decrease or increase in the expression of connexin 43 in the heart muscle. Additional studies are needed to determine the role of connexin 43 in other parts of the cardiac muscle and further research is needed to determine the other effects of tannic acid regardless of its antioxidant effect.