Continuous oral bromodeoxyuridine (BrdU) administration was used for the non-invasive measurement of the in vivo cell cycling characteristics of hematopoietic stem and progenitor cell populations of increasing maturity, isolated on the basis of their relative levels of Rhodamine 123 (Rh) and Hoechst 33342 (Ho) fluorescence. The results showed that whereas primitive hematopoietic stem cells (PHSCs) are hierarchically ordered on the basis of quiescence, the most primitive of these, characterized by their Rh/Ho (dull) phenotype and their capacity for long-term hematopoietic reconstitution, are not dormant, but cycle slowly in normal steady-state bone marrow (BM). Cell cycle analysis showed that 30+/-7% of Rh/Ho (dull) PHSCs had cycled and incorporated BrdU following continuous administration over 1 week, whereas 60+/-14% and 89+/-3% of these cells were BrdU positive at 4 and 12 weeks, respectively. Linear regression analysis of these data showed that Rh/Ho (dull) PHSCs cycle with an average turnover time of 4.3 weeks (30 days), and a t1/2 (time to 50% cycled) of 2.75 weeks (19 days).