Objective To assess changes in plasma exosome levels and protein content in mice after long-term exercise. Methods We subjected 9-month-old adult C57BL/6J mice to daily treadmill running exercise for 4 weeks prior to the isolation of blood-derived exosomes. Exosomal proteins were identified using mass spectrometry. Results Extracellular bodies were successfully isolated from mouse blood. Protein levels were altered in blood-derived exosomes after chronic treadmill exercise. Levels of the secretagogue secretogranin 2 were markedly elevated in exercise-induced exosomes. Conclusion Our data suggest that levels of secretogranin 2 were increased in mouse exosomes following chronic treadmill exercise. We conclude that exercise increases exocrine secretion of secretogranin 2.