We searched for morphological evidence to support the hypothesis that stem cells are responsible for renal tubular cell proliferation. The rationale of the study was that if proliferation relies on progenitors, mitotically active cells should be less differentiated than their neighbors. As the retention of the thymidine analog BrdU has been the only approach employed to identify stem cells in the kidney up to now we additionally characterized BrdU-retaining cells. Rat kidneys were fixed by perfusion. Cycling cells identified by mitotic figures or the expression of the proliferating cell nuclear antigen (PCNA) were examined by light microscopy and electron microscopy as well as immunofluorescence for four differentiation markers. Newborn rats were injected with BrdU in order to detect label-retaining cells. After a period of 8, 14 and 35 weeks the kidneys were examined for BrdU by immunofluorescence and the four differentiation markers mentioned above. All cycling cells showed the same degree of differentiation compared to non-cycling cells. Most of the detected label-retaining cells were differentiated. We conclude that cycling cells in tubules of the healthy kidney are differentiated and that the retention of label is not a criterion to identify stem cells in renal tubules.