An analysis has been made of the experimental conditions under which overgrowth can be produced in the brains of chick embryos. A total of 122 individuals were obtained from six series of embryos subjected to varying degrees of surgical manipulation involving the anterior rhombomere and/or its substratum. The results indicate that in addition to the cases of overgrowth obtained by the standard technique of removing the entire anterior rhombomere, overgrowth could also be successfully induced when a portion of the chordamesoderm underlying the anterior rhombomere was either removed or merely dissociated from the neural tissue, while leaving the latter relatively intact. In embryos in which the roof and sides of the anterior rhombomere were removed, overgrowth could not be obtained in the absence of damage to the substratum.