Hormone oxytocin belongs to the factors activated by physical and mental stress stimuli. Some reports suggest possible role of oxytocin in the control of cardiovascular functions. This study was designed to verify the hypothesis that oxytocin release and content increase in response to prolonged physical activity in a model of voluntary wheel running. Adult Sprague-Dawley rats were exposed to voluntary running for three weeks. Animals were housed under normal conditions in standard cages or in cages with a free access to running wheel. Oxytocin levels in plasma, in the posterior pituitary and in the heart were analyzed by a radioimmunoassay. No significant changes were observed in plasma oxytocin while the content of oxytocin in the posterior pituitary decreased in response to voluntary running. Vasopressin concentrations failed to be modified by voluntary wheel running. The suggestion that the rise in oxytocin levels in the blood is manifested during the night (the time of maximal intensity of running) was excluded in experiments performed during the dark phase of the day. As to the heart, oxytocin levels were found to be present in both ventricles and atria. Concentration of oxytocin in heart atrium (right auricle) decreased in voluntary running animals. Thus, oxytocin may be involved in cardiovascular regulations during voluntary exercise. Supported by SP51/0280800/0280802, partially by VEGA 5064 and ESF.