Mice homozygous for the recessive gene achondroplasia (cn) aged 16 and 17 days and some homozygotes aged 22-34 days have disruptions in the growth of the proximal tibial growth plate which are due solely to reduced hypertrophic cell height. A second class of homozygote, distinguishable at 22 days, has a greater disruption due to much reduced hypertrophic cell height, reduced labelling index and reduction of the number of cells in the effective proliferative zone.