In newborn mice of the control [C3H/HeJ (C3H)] and monoamine oxidase A-deficient (Tg8) strains, in which levels of endogenous serotonin (5-HT) were drastically increased, we investigated how 5-HT system dysregulation affected the maturation of phrenic motoneurons (PhMns), which innervate the diaphragm. First, using immunocytochemistry and confocal microscopy, we observed a 5-HT2A receptor (5-HT2A-R) expression in PhMns of both C3H and Tg8 neonates at the somatic and dendritic levels, whereas 5-HT1B receptor (5-HT1B-R) expression was observed only in Tg8 PhMns at the somatic level. We investigated the interactions between 5-HT2A-R and 5-HT1B-R during maturation by treating pregnant C3H mice with a 5-HT2A-R agonist (2,5-dimethoxy-4-iodoamphetamine hydrochloride). This pharmacological overactivation of 5-HT2A-R induced a somatic expression of 5-HT1B-R in PhMns of their progeny. Conversely, treatment of pregnant Tg8 mice with a 5-HT2A-R antagonist (ketanserin) decreased the 5-HT1B-R density in PhMns of their progeny. Second, using retrograde transneuronal tracing with rabies virus injected into the diaphragm of Tg8 and C3H neonates, we studied the organization of the premotor network driving PhMns. The interneuronal network monosynaptically connected to PhMns was much more extensive in Tg8 than in C3H neonates. However, treatment of pregnant C3H mice with 2,5-dimethoxy-4-iodoamphetamine hydrochloride switched the premotoneuronal network of their progeny from a C3H- to a Tg8-like pattern. These results show that a prenatal 5-HT excess affects, via the overactivation of 5-HT2A-R, the expression of 5-HT1B-R in PhMns and the organization of their premotor network.