1. The activity of locus coeruleus (LC) neurones (n = 126) was examined in whole-cell (conventional and amphotericin B-perforated patch) recordings, and the relationship of this activity to the respiratory discharge recorded on the C4 or C5 phrenic nerve roots was determined at different CO2 concentrations (2 and 8 %; bath pH 7. 8 and 7.2) in the in vitro brainstem-spinal cord preparation of the neonatal rat (1-5 days old). 2. In most neurones (n = 105) ongoing activity was modulated at respiratory frequency. Typically, this consisted of a phase of depolarization and increased discharge frequency synchronous with the phrenic burst, followed by a phase of hyperpolarization and inhibition of discharge (n = 94 of 105). The incidence of respiratory modulation decreased from 91 % on P1 to 57 % on P5. 3. Bath application of the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 5 microM) or the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV; 100 microM) abolished both phases of respiratory modulation. The hyperpolarizing phase alone was abolished by the adrenoceptor antagonists idazoxan (5 microM) or phentolamine (0.8 microM). These results indicate that excitatory amino acid pathways are involved in the transmission of both the excitatory and inhibitory components and that the latter involves in addition an alpha2-adrenoceptor-mediated pathway. 4. Increasing the CO2 concentration from 2 to 8 % resulted in a shortening of expiratory duration and weakening or loss of respiratory-phased inhibition; this was accompanied by depolarization, increased discharge frequency and, in those neurones where they were initially present (60 %), an increase in the frequency of subthreshold membrane potential oscillations. The depolarizing response was retained in the presence of tetrodotoxin (TTX, 0.2-1.0 microM). 5. These results indicate that in this neonatal preparation LC neurones form part of the synaptically connected brainstem respiratory network, and that the LC constitutes a site of CO2- or pH-dependent chemoreception.