Growth of femora and tibiae has been measured in mice carrying three distinct chondrodystrophic mutants (achondroplasia cn, brachymorphic bm and stumpy stm) aged 6--128 days, and in normal litter-mates. cn and bm resemble each other in growing slowly until the time of weaning, when growth is interrupted, stm grows strongly at first, but stops at around 14 days. The significance of these findings is discussed.