Diploma thesis title: The effect of metformin on mitochondrial respiration in the heart. Objectives: To examine the effect of metformin on mitochondrial respiratory chain in rat myocardium. Materials and methods: A controlled experiment on 10 rats of Sprague Dawley strain was conducted. The rats were raised in a controlled environment at the animal facility of the University of Split School of Medicine. The effect of metformin was examined in vitro using rat left ventricular homogenate. Oxygen sensitive electrode was used to examine mitochondrial respiration which was stimulated by using two different metabolic substrates. By adding pyruvate and malate, respiration mediated by the complex I of the respiratory chain has been recorded. Addition of succinate (in the presence of rotenone) enabled measurement of respiration involving complex II of the respiratory chain. Two different concentrations of metformin were tested. Concentration of 30 µmol/l is usually detected in plasma of patients taking metformin in their therapy. The other tested metofrmin concentration was 500 µmol/l. Results: Metformin applied in the concentration of 30 µmol/l did not significantly affect mitochondrial oxygen consumption except after 120 minutes of application when measured respiration was 722 % of basal. With 500 µmol/l metformin, 5 minutes after applying the drug no significant effect on mitochondrial respiration in vitro was detected. However, from 30. to 120. minute, a statistically significant decrease in mitochondrial oxygen consumption was observed. From initial 874 % of basal respiration at 30. minute, in the presence of metformin the respiration has fallen to 57±7% of basal in 120. minute (compared with respiration in control chamber which was 924 % at 30. minute and 77±11 % at 120. minute). Neither of the used metformin concentrations exhibited any inhibitory effect on mitochondrial respiration in the presence of complex II substrate. After applying the drug in the concentration of 30 µmol/l the detected respiration was 63±10% of basal compared with 64±10% of basal measured in control chamber. Also, after applying metformin in the concentration of 500 µmol/l, the measured respiration was 55±9% of basal compared with 59±11% which was detected in the control chamber. Conclusions: Metformin applied in vitro in the concentration of 30 µmol/l does not exhibit an inhibitory effect on mitochondrial respiration. Metformin applied in vitro in the concentration of 500 µmol/l demonstrates a time dependent, progressive inhibitory effect on mitochondrial respiration which is mediated trough complex I of the respiratory chain.