As medullary and spinal respiratory activities coexist in the in vitro brainstem spinal cord preparation from newborn rats previously deeply anaesthetized with diethyl ether, we sought evidence for reciprocal interactions between the medullary and spinal respiratory generators. A descending influence of medullary onto spinal respiratory generators has been characterized by the constant onset latency of the spinal burst within the corresponding medullary respiratory cycle. Non-harmonic (29%) and harmonic (9%) correlations were observed between the two phenomena, but correlations could also be low (27%) or absent (35%). Variations in the pH or FCO2 of the artificial cerebrospinal fluid were able to induce or suppress this medullary control of spinal respiratory activity. An ascending relationship between spinal and medullary respiratory generators has been revealed as a systematic variation of the medullary period due to resetting. Depending on the timing of the spinal respiratory activity, the medullary burst could be either advanced through a shortened medullary period, or delayed through a lengthened one. In conclusion, it was possible to show the existence of reciprocal relationships between the medullary and spinal respiratory generators in the in vitro brainstem spinal cord preparation from newborn rats. These results are interpreted in relation to the central interactions between locomotion and respiration in mammals which occur during rapid propulsion and which could be mediated by the spinal respiratory generator.