Apelin peptides are now known to be endogenous ligands at the orphan G-protein coupled receptor, APJ. Apelin and its receptor have been found in the brainstem and shown to have a role in haemodynamic homeostasis when injected intravenously. The physiological role of this peptide and its receptor centrally are yet to be elucidated. In this study, urethane anaesthetised, paralysed and ventilated male Sprague-Dawley rats (350-450 g, n=4) were used to investigate the action of apelin-13 microinjected directly into the nucleus tractus solitarius (NTS) and the rostral ventrolateral medulla (RVLM) on arterial pressure and phrenic nerve activity. Apelin-13 microinjections (4 mmol/l, 50 nl) into the NTS resulted in either apnea or decreased phrenic nerve discharge amplitude by up to 30%. In the RVLM, apelin-13 caused either a 100-200% increase, or a 10-30% increase in phrenic nerve discharge amplitude depending on the exact site of injection. Increases of 10-20 mm Hg in arterial pressure were also evoked from both the NTS and the RVLM. These data suggest a role for apelin-13 in arterial pressure and respiratory control in the NTS and the RVLM.