Down syndrome cell adhesion molecule (DSCAM) is a neural adhesion molecule that plays diverse roles in neural development. We disrupted the Dscam locus in mice and found that the null mutants (Dscam(-/-)) died within 24 h after birth. Whole-body plethysmography showed irregular respiration and lower ventilatory response to hypercapnia in the null mutants. Furthermore, a medulla-spinal cord preparation of Dscam(-/-) mice showed that the C4 ventral root activity, which drives diaphragm contraction for inspiration, had an irregular rhythm with frequent apneas. Optical imaging of the preparation using voltage-sensitive dye revealed that the pre-inspiratory neurons located in the rostral ventrolateral medulla and belonging to the rhythm generator for respiration, lost their synchroneity in Dscam(-/-) mice. Dscam(+/-) mice, which survived to adulthood without any overt abnormalities, also showed irregular respiration but milder than Dscam(-/-) mice. These results suggest that DSCAM plays a critical role in central respiratory regulation in a dosage-dependent manner.