We have employed a simple and robust non-invasive method of continuous in vivo long-term bromodeoxyuridine (BrdU) labelling to analyse lung mesenchymal stromal cell turnover in adult mice in the steady state. Mathematical modelling of BrdU uptake in flow cytometrically sorted CD45^negCD31^negSca-1^pos lung cells following long-term feeding of BrdU to mice in their drinking water reveals that lung mesenchymal stromal cells cycle continuously throughout life. Analysis of BrdU incorporation during long-term feeding, and during chasing (delabeling) following replacement of BrdU-water with normal water shows that the CD45^negCD31^negSca-1^pos lung mesenchymal cell compartment turns over at a rate of approximately 2.26% per day with a time to half-cycled of the compartment of 44 days, with an estimated cell proliferation rate of 0.004/day, and a cell death rate of 0.018/day.