The influence on the breathing pattern of the activation of serotonin receptors belonging to the subtypes 2(A) and 2(C) (5-HT(2A/2C)) has been assessed in newborn and adult conscious rats. Rats were given an acute intraperitoneal dose of the agonist DOI (1-(2.5-dimethoxy-4-iodophenyl)-2-aminopropane; 5 mg/kg). In newborns, DOI elicited a long-lasting respiratory depression by decreasing both tidal volume and respiratory frequency. In adults, DOI retained a depressant influence, although attenuated, on tidal volume. In contrast, it elicited an increase in respiratory frequency. In separate subsets of newborn and adult rats, immunohistochemistry has been used to monitor c-fos expression induced by DOI in the medullary and pontine regions involved in respiratory control. Counts of immunoreactive neurons indicated a marked increase in the neuronal populations activated in the adult compared to the newborn rat. The response to both experimental factors (newborn vs. adult controls) and drug (injected vs. control age-matched rats) were more pronounced in mature animals. Among developmental changes in the pattern of labeling, DOI elicited Fos expression in the adult but not in the neonate in the ventrolateral subnucleus of the nucleus of the solitary tract, the parabrachial area and the Kölliker-Fuse nucleus. This finding suggested that changes in the respiratory response to DOI might at least partly depend on maturational events within networks involved in the modulation of respiratory timing.