We have shown previously that the inactivation of the zinc finger gene Krox-20 affects hindbrain segmentation, resulting in the elimination of rhombomeres 3 and 5. We demonstrate here that Krox-20 homozygous mutant mice exhibit abnormally slow respiratory and jaw opening rhythms, indicating that a modification of hindbrain segmentation influences the function of neuronal networks after birth. Central neuronal networks that control respiratory frequency are made predominantly depressant by the elimination of a previously undescribed rhythm-promoting system. Recordings of rhythmic activity from the isolated hindbrain following progressive tissue transections indicate that the reorganization takes place in the caudal pontine reticular formation. The newborn (PO) Krox-20-/- mice, in which apneas are ten times longer than in wild-type animals, may be a valuable model for the study of life-threatening apneas during early infancy.