Background/aim: Irisin, a newly identified exercise-derived myokine, has been found involved in a peripheral vasodilator effect. However, little is known regarding the potential vascular activity of irisin, and the mechanisms underlying its effects on vascular smooth muscle have not been fully elucidated. This study was aimed to investigate the effects of irisin on vascular smooth muscle contractility in rat thoracic aorta, and the hypothesis that protein kinase C (PKC) may have a role in these effects. Materials and methods: Isometric contraction-relaxation responses of thoracic aorta rings were measured with an isolated organ bath model. The steady contraction was induced with 10 mu M phenylephrine (PHE), and then the concentration-dependent responses of irisin (0.001-1 mu M) were examined. The time-matched vehicle control (double distilled water) group was also formed. To evaluate the role of PKC, endothelium-intact thoracic aorta rings were incubated with 150 nM bisindolylmaleimide I (BIM I) for 20 min before the addition of 10 mu M PHE and irisin. Also, a vehicle control group was formed for dimethyl sulfoxide (DMSO). Results: Irisin exerted the vasorelaxant effects at concentrations of 0.01, 0.1, and 1 mu M compared to the control group (p < 0.001). Besides, PKC inhibitor BIM I incubation significantly inhibited the relaxation responses induced by varying concentrations of irisin (p: 0.000 for 0.01 mu M; p: 0.000 for 0.1 mu M; p: 0.000 for 1 mu M). However, DMSO, a solvent of BIM I, did not modulate the relaxant effects of irisin (p > 0.05). Conclusion: In conclusion, physiological findings were obtained regarding the functional relaxing effects of irisin in rat thoracic aorta. The findings demonstrated that irisin induces relaxation responses in endothelium-intact thoracic aorta rings in a concentration-dependent manner. Furthermore, this study is the first to report that irisin-induced relaxation responses are regulated probably via activating signaling pathways implicating PKC.