A polarizing activity in the developing vertebrate limb bud has been implicated in the control of the morphogenesis of its anteroposterior axis. The results of an earlier experiment suggest that there is a gradient of a similar morphogenetic activity in the 4-day chick wing, with a high level of activity along the posterior border, no detectable activity at the anterior border, and an intermediate level of activity in the center, i.e., about halfway between the anterior and posterior borders. The experiments reported here show that this activity disappears from the center of the limb after placing an impermeable barrier posterior to the center, but not after placing the barrier anterior to the center. A porous barrier placed posterior to the center does not alter the activity in the center of the limb. These results lead us to suggest that the gradient of morphogenetic activity in the chick wing is the result of the movement (possibly by diffusion) of a factor(s) from a source in the posterior region of the wing.