Notch signalling, via its downstream mediators HES1 and HES5, regulates development of several different tissues. In vitro studies suggest that these genes are also involved in chondrogenesis and endochondral bone formation. In order to investigate the importance of HES1 and HES5 for these developmental processes, mice lacking chondrogenic expression of HES1 and HES5 were constructed by interbreeding HES5(-/-) mice homozygous for the floxed HES1 allele (HES1(flox/flox)) with COL2A1-Cre transgenic mice, creating conditional HES1;HES5 double mutant mice. The formation of cartilage and endochondral bone was studied in these mice using histological and immunohistochemical stainings, including Alcian Blue van Gieson, Safranin-O, modified Mallory Aniline Blue, tartrate-resistant acid phosphatase and collagen type II stainings. The mice were also studied using several different morphometrical analyses and the differentiation potential of the chondrocytes was evaluated in vitro. Unexpectedly, the conditional HES1;HES5 double mutant mice did not display impaired development of cartilage or endochondral bone. Lack of altered phenotype in the conditional HES1;HES5 double mutant mice can be explained either by the HES1 and HES5 genes not being involved in cartilage and endochondral bone development or by functional redundancy between the genes belonging to the family of HES genes: that is, disruption of one gene could be compensated for by the activity of another. Our results further shed light on the compensatory reserves available during the developing cartilage and bone.