Thirty male adult rats, weighing 250–300 g, were injected with 0.5 μCi [³H]‐thymidine. The rats were killed in groups of five, at the following times: 1 h, 14, 30, 60, 90 and 120 days. The livers were fixed in formalin, embedded in paraffin and cut into 5‐μm‐thick sections, which were then dipped into liquid emulsion for autoradiography. Bile ducts were evaluated in all portal tracts of a section. Obliquely cut bile ducts were excluded. The number of cells paving the duct lumen was defined as bile duct class. Class is roughly proportional to bile duct size. One hour after labelling, labelling index was 3.39 ± 0.06% (the second number is one standard error of the mean). With time it declined, reaching 0.22 ± 0.15 on day 60. Initially labelled cells were found in narrow bile ducts and the mean labelled cell was located at class 7.48 ± 0.63. With time, labelled cells proceeded into wider bile ducts advancing daily 0.122 ± 0.025 classes. Intra‐hepatic bile duct epithelium renews its cells continuously in the same way as epidermis or gastro‐intestinal mucosa do. It consists of two kinetic compartments, a progenitor (P) and a functional (Q) compartment. P feeds Q with cells.