A study was undertaken to analyze the spatio-temporal pattern of mesenchymal cell proliferation in the developing palate of quail. Quail embryos were grown in shell-less culture. The developing palates were labelled with 3H-thymidine between culture days 2-6 (which corresponded in vivo incubation days 5-9), and processed for light microscopic autoradiography. Percent labelled mesenchymal cells were determined. The data showed that, as in mammals, a high rate of random cell proliferation in mesenchyme was a major component of early palate development in quail. As the palate morphogenesis advanced, the rate of cell proliferation declined. Segmental analysis, however, indicated that, in contrast to mammals, the mesenchymal cell proliferation rates continually changed in various regions of quail palate during morphogenesis. It was suggested that the spatio-temporal changes in the distribution of dividing cells may reflect differences in the timings of cell cycles between various segments, thus resulting in a heterogeneous population of cells in the developing palate of quail. Further, the differences in the segmental pattern of cell proliferation between birds and mammals may form the basis for differences in the morphogenesis of their palates.