Cyclic GMP-dependent protein kinase type I (cGKI) is a key signaling intermediate important for synaptic potentiation in the hippocampus and cerebellum, but its expression and function in cortical development have not been elucidated. The expression of cGKI in the developing mouse neocortex was evaluated by immunofluorescence labeling, and effect of cGKI deletion on cortical development was studied in adult cGKI knockout mice. cGKI was expressed at highest levels at embryonic stages in young neurons and radial glial fibers, corresponding to the major period of radial migration and laminar development of pyramidal neurons (embryonic day E13.5-E14.5), declining upon maturation (E17.5-postnatal day P28). The cerebral cortex of homozygous null mutant mice lacking cGKI exhibited heterotopic collections of neurons in the upper cortical layers and abnormal invaginations of layer I, in accord with a neuronal migration or positioning defect. Some cGKI mutant mice displayed defects in midline development resulting in partial fusion of cerebral hemispheres with adjacent neuronal heterotopias. Apical dendrites of cortical pyramidal neurons were misoriented in the cerebral cortex of cGKI null mutants, as shown in reporter mice expressing yellow fluorescent protein in layer V pyramidal neurons and by Golgi impregnation. These results demonstrate a role for cGKI signaling in cortical development related to neuronal migration/positioning that is important for dendritic orientation and connectivity.