In order to determine whether the serotonergic modulation of the central respiratory activity previously reported in neonatal rats occurs in species other than the rat, we performed identical in vitro experiments on the neonatal mouse to those performed on the neonatal rat. The effects of adding serotonin (5-hydroxytryptamine, 5-HT) and related agents to the superfusate suggested that the respiratory rhythm generator undergoes an excitatory modulation via medullary 5-HT1A receptors. Upon applying the drugs to the spinal cord alone, 5-HT was found to have a dual effect on phrenic motoneuron firing: (i) a facilitatory effect mediated by 5-HT2A receptors and (ii) a depressive effect on their inspiratory discharge mediated by non-5-HT1A, non-5-HT2A, non-5-HT3 receptors, possibly of the 5-HT1B subtype. It was therefore concluded that serotonin modulates the neonatal central respiratory activity in mice as well as in rats, and that similar 5-HT receptor subtypes are involved in this process in both species.