AIM: Botulinum toxin type A (botox) is an acetylcholine release inhibitor and a neuromuscular blocking agent indicated in adult patients for the temporary improvement in the muscle. There isn’t sufficient number of studies with botulinum toxin type A about cardiac tissues. We aimed to investigate the effects of botulinum toxin A on human atrium contraction. METHODS: Human atrium tissues (n=12) were taken from patients who underwent coronary bypass surgery. The patients’ ages were between 50 to 75. All tissues were placed in organ baths containing Krebs solution, thermoregulated at 37 0C and aerated (95 % O2 and 5% CO2). Changes in isometric tensions were recorded using a four channel force displacement transducer. All tissues washed for 3 hours in order to diminish the effects of anaesthetic agents. Adrenaline (10-1 M) was administered in tissue cabs to induce isometric contractions. 100 units vacuum-dried powder Botox was diluted by 5ml 0.9% NaCl solution. Botox doses of o.1mL (4 units), 0.2 mL (8 units), 0.3 mL (12 units) and 5mL (20 units) were added in organ baths and contractions were recorded accordingly. Contraction with measurements were used as contraction parameters. Friedman and Kruskal Wallis tests were used for statistical evaluation. RESULTS AND CONCLUSIONS: After last dose of botulinum toxin type A (0.5mL, 20 units) was given, significant inhibition of spontaneous contractions was noticed (p<0.05). Vasodilatation of atrium suggests that contraction of atrium and ejection fraction decrease. Further studies are needed to clarify the mechanism of Botulinum Toxin Type A.