The developing vertebrate nervous system arises from ectoderm in response to inductive signals from the dorsal mesoderm, or Spemann organizer. It displays pronounced anteroposterior (AP) pattern, but the mechanism that generates this pattern is poorly understood. We demonstrate that the inducing ability of dorsal mesoderm is regionalized along the AP axis at the early gastrula stage, using the homeodomain-encoding genes Xanf-2 and en-2 as markers of anterior and mid-neural pattern, respectively. In addition, we show that changing the size ratio of posterior dorsal mesoderm to responding ectoderm affects the type of AP pattern induced. A low ratio leads to induction of anterior neural pattern, while a high ratio leads to expression of only mid-neural pattern. These and other results indicate that a quantitative mechanism specifies AP neural pattern.