Bone morphogenetic proteins are members of the transforming growth factor beta (TGF beta) superfamily which are involved in a range of developmental processes including modelling of the skeleton. We show here that Bmp-2 is expressed in mesenchyme surrounding early cartilage condensations in the developing chick limb, and that Bmp-4 is expressed in the perichondrium of developing cartilage elements. To investigate their roles during cartilage development, BMP-2 and BMP-4 were expressed ectopically in developing chick limbs using retroviral vectors. Over-expression of BMP-2 or BMP-4 led to a dramatic increase in the volume of cartilage elements, altered their shapes and led to joint fusions. This increase in volume appeared to result from an increase in the amount of matrix and in the number of chondrocytes. The latter did not appear to be due to increased proliferation of chondrocytes, suggesting that it may result from increased recruitment of precursors. BMP-2 and BMP-4 also delayed hypertrophy of chondrocytes and formation of the osteogenic periosteum. These data provide insights into how BMP-2 and BMP-4 may model and control the growth of skeletal elements during normal embryonic development, suggesting roles for both molecules in recruiting non-chondrogenic precursors to chondrogenic fate.