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Background: Skeletal muscles are postulated to be a potent regulator of systemic nitric oxide homeostasis. In this study, we aimed to evaluate the impact of physical training on the heart and skeletal muscle nitric oxide bioavailability (judged on the basis of intramuscular nitrite and nitrate) in rats., Methods and Results: Rats were trained on a treadmill for 8 weeks, performing mainly endurance running sessions with some sprinting runs. Muscle nitrite (NO 2 - ) and nitrate (NO 3 - ) concentrations were measured using a high-performance liquid chromatography-based method, while amino acids, pyruvate, lactate, and reduced and oxidized glutathione were determined using a liquid chromatography coupled with tandem mass spectrometry technique. The content of muscle nitrite reductases (electron transport chain proteins, myoglobin, and xanthine oxidase) was assessed by western immunoblotting. We found that 8 weeks of endurance training decreased basal NO 2 - in the locomotory muscles and in the heart, without changes in the basal NO 3 - . In the slow-twitch oxidative soleus muscle, the decrease in NO 2 - was already present after the first week of training, and the content of nitrite reductases remained unchanged throughout the entire period of training, except for the electron transport chain protein content, which increased no sooner than after 8 weeks of training., Conclusions: Muscle NO 2 - level, opposed to NO 3 - , decreases in the time course of training. This effect is rapid and already visible in the slow-oxidative soleus after the first week of training. The underlying mechanisms of training-induced muscle NO 2 - decrease may involve an increase in the oxidative stress, as well as metabolite changes related to an increased muscle anaerobic glycolytic activity contributing to (1) direct chemical reduction of NO 2 - or (2) activation of muscle nitrite reductases.