Pax3 has been used as a valuable marker in research aimed at understanding tissue interactions involved in trigeminal ophthalmic (opV) placode development. While Pax3 expression coincides with opV neuron specification, the function of Pax3 in these cells has not previously been investigated. Splotch mutant mice (which lack Pax3) have a reduced trigeminal ganglion; however it is not clear whether this reduction is due to neural crest or placode cells. We have used electroporation in the chick model system to block or ectopically express Pax3 at key times in opV placode development. Using several markers of placode cell differentiation, we have determined the experimental effects manipulating Pax3. Blocking placodal Pax3 with gene specific morpholinos resulted in a loss of migratory placode cells, and a downregulation of all opV placode markers in targeted cells. Ectopic expression of Pax3, either within the placode domain or in adjacent cranial ectoderm, resulted in the upregulation of some but not all placode markers. We conclude that opV placodal Pax3 expression is required for normal placode cell development, and hypothesize that its expression must be tightly regulated in order for placode cells to fully differentiate. The precise role of Pax3 and Pax7 in the restriction and differentiation of dorsal interneuron progenitors has been difficult to assess due to the many additional factors involved in specification and patterning of the neural tube. We have used electroporation in the chick model system to ectopically express Pax3 and Pax7 unilaterally in the neural tube. Using several markers for differentiation of ventral and dorsal neuronal progenitors, we have experimentally determined the effects of Pax3 and Pax7 ventrally and dorsally. Ectopic expression of these transcription factors in the ventral neural tube resulted in the loss of motorneurons. Though mis-expression did not qualitatively affect commissural neurons as assayed by neurofilament staining, ectopic expression of Pax3 and Pax7 in the dorsal neural tube stopped dorsal interneuron progenitors from differentiating. We conclude that Pax3 and Pax7 expression is sufficient to restrict ventral neuron identity. We also hypothesize that downregulation of these transcription factors in the dorsal neural tube is required for normal dorsal interneuron differentiation.