Bone morphogenetic proteins (BMPs) regulate many aspects of development including skeletogenesis. Here, we examined the response of neural crest-derived cells to ectopic BMP signaling by infecting avian embryos with retroviruses encoding Bmp-2 or Bmp-4 at various times of development. Infection at stages 10 and 15 transformed large areas of the skull into cartilage by day 13. At this time cartilage condensations were still forming, which revealed the presence of uncommitted mesenchymal cells. By day 19, hypertrophic chondrocytes were present in the cartilage possibly due to changes in the perichondrium that relieved repression on hypertrophy. While these cells expressed Sox9, Collagen-2, Runx2, Ihh, Noggin, and Collagen-10, cartilage was not replaced by bone. Whether this is an intrinsic property of the skull cartilage, or results from sustained Bmp signaling is not known. Developmental Dynamics 237:3727–3737, 2008. © 2008 Wiley-Liss, Inc.