The labelling index (LI) of the synoviocytes of the lining-layer cells and of the underlying connective-tissue cells was determined in the synovial membrane of 5-, 10-, 21-, 30-, and 55 day-old mice, following a pulse labelling with [3H]-thymidine. Both the synoviocytes and underlying connective-tissue cells incorporated [3H]-thymidine within 1 h after injection. No significant difference was observed between the LI of these two regions of the synovial membrane. The results indicate that synoviocytes are capable of cell division even in non-pathological conditions.