To investigate whether mesodermal expansion in the area vasculosa is caused by tension produced by outward migration of cells either in the somatic mesoderm or at the mesodermal edge on an ectodermal substratum, stage 18--20 embryos were transferred to a culture dish. There mesodermal expansion proximal to an arrested edge could be compared with that proximal to a moving edge by measuring the amount of vascular elongation occurring in each. A proximo-distal gradient in vascular elongation rate was detected both in normal embryos in ovo and in explants. This gradient was reversed following arrest of the edge, and the rate of vascular elongation proximal to the arrested edge decreased to 60--70% of that proximal to a moving edge. Nearly all of the mesoderm producing this expansion was located in the proximal two-thirds of the area vasculosa, where vascular elongation rate on the stopped side of the explant was not significantly different from that on the moving side. Similar results were obtained in the absence of the ectoderm, and when liquid culture medium was used instead of semisolid medium. It is concluded that tensile force derived from mesodermal migration plays no role in expansion of the proximal two-thirds of the area vasculosa mesoderm.