The role of a notochord fragment on the origin of an additional floor plate area in the neural tube is investigated by quantitative morphological methods. In 1.5 to 2 day chick embryos a notochordal fragment was implanted in close apposition to the lateral wall of the neural groove in the region between prospective wing and leg bud. At 4 days, adjacent to the implant a distinct area of the neural wall was present, which resembled the natural floor plate with respect to its thickness, the abluminal location of elongated nuclei and the absence of neuroblasts. The mitotic density of this area was reduced. This "additional floor plate" was distinct when the experiment was performed at 1.5 days but was hardly recognizable when it was carried out at 2 days. From these results it is concluded that a) the notochord induces floor plate like structures and diminishes proliferation, and b) that the period of floor plate induction by the notochord is very restricted.