The involvement of the rostral ventro-lateral medulla (RVLM) in respiratory rhythm genesis was analysed on brain stem-spinal cord preparations from newborn and fetal rats in which the ability to generate central respiratory activity in vitro persists. The respiratory rhythm (around 5 per min) was stable in preparations from newborn and pre-term fetal (D20-21) rats but very variable in young fetuses (D18). In newborn and D20-21 fetal rats, RVLM electrical stimulation delivered during mid-expiration initiated premature inspiration while RVLM electrolytic lesions suppressed the respiratory rhythm. In D18 fetuses, RVLM stimulation had no effect and strong stimulations evoked only diffuse activation. Extracellular recordings of the activity of 423 RVLM neurons showed that this area contained numerous inspiratory neurons in all the age groups studied. Low Ca(2+)-high Mg2+ medium bathing (assumed to block synaptic transmission) abolished the inspiratory bursts in the cervical roots and most of the 99 RVLM inspiratory neurons investigated. In newborn and D20-21 rats, however, 7 of the 68 RVLM inspiratory neurons tested behaved like respiratory pacemakers, since they continued to fire with a bursting pattern, while in the D18 preparations, none of 31 did so. These experiments confirm that the RVLM is a crucial site in respiratory rhythm genesis in newborn rats, and suggest that this may also be the case in D20-21 fetuses, but probably not in D18 fetuses.