The aim of this thesis was to demonstrate the potential negative effect of the tannic acid on the expression of connexin 45 in rat kidney damaged by the chronic hyperglycemia, due to a need for critical assessment of the positive effects of the use of tannic acid in the treatment of many diseases. Twelve male Wistar rats were used in the trial over a period of twenty weeks. The rats were divided into three groups of four rats. The groups were formed based on beverages consumed by the rats. At the end of this 20-week experiment it was observed that the group of rats that consumed sucrose in drinking water (30%) had a significant increase in blood glucose level compared to the control group which consumed drinking water. In the third group, where tannic acid along with the sucrose (0,1% of tannic acid; Sah/TK group) was added in a drinking water, blood glucose levels were less increased than in the group that consumed sucrose in water. Also consumption of tannic acid (0,1%) had a positive effect on the weight reduction. Pathohistologically, we observed damage of the kidneys caused by the hyperglycemia, while damage was more expressed in the Sah/TK group. Analysis of the microphotographs showed that hyperglycemia caused reduced expression of connexin 45 in the proximal tubules of the kidney, and the addition of the tannic acid made the reduction statistically significant (p <0.05). Based on the results, we concluded that the tannic acid in combination with the chronic hyperglycemia, in the dosage of 0,1% in drinking water, damages the kidneys. We also concluded that tannic acid in combination with the chronic hyperglycemia significantly decreases the expression of the connexin 45 in the proximal tubules of the rat kidney.