The c-Jun NH(2)-terminal protein kinase (JNK), which belongs to the mitogen-activated protein kinase family, plays important roles in a broad range of physiological processes. JNK is controlled by two upstream regulators, mitogen-activated protein kinase kinase (MKK) 7 and MKK4. To elucidate the physiological functions of MKK7, we used Nestin-Cre to generate a novel mouse model in which the mkk7 gene was specifically deleted in the nervous system (Mkk7(flox/flox) Nestin-Cre mice). These mice were indistinguishable from their control littermates in gross appearance during embryogenesis but died immediately after birth without breathing. Histological examination showed that the mutants had severe defects in brain development, including enlarged ventricles, reduced striatum, and minimal axon tracts. Electron microscopy revealed abnormal accumulations of filamentous structures and autophagic vacuoles in Mkk7(flox/flox) Nestin-Cre brain. Further analysis showed that MKK7 deletion decreased numbers of TAG-1-expressing axons and delayed neuronal migration in the cerebrum. Neuronal differentiation was not altered. In utero electroporation studies showed that contralateral projection of axons by layer 2/3 neurons was impaired in the absence of MKK7. Moreover, MKK7 regulated axon elongation in a cell-autonomous manner in vivo, a finding confirmed in vitro. Finally, phosphorylation levels of JNK substrates, including c-Jun, neurofilament heavy chain, microtubule-associated protein 1B, and doublecortin, were reduced in Mkk7(flox/flox) Nestin-Cre brain. Our findings demonstrate that the phenotype of Mkk7(flox/flox) Nestin-Cre mice differs substantially from that of Mkk4(flox/flox) Nestin-Cre mice, and establish that MKK7-mediated regulation of JNK is uniquely critical for both axon elongation and radial migration in the developing brain.