Many in vitro studies of sympathetic neuronal activity (including cellular and nerve recordings) have been done in preparations from neonatal rats. However, it has been difficult to extrapolate these findings to mature animals, because there have been no recordings of sympathetic nerve activity (SNA) in functionally intact, neonatal rats. Hence we have compared spontaneous and reflex-evoked SNA in neonate (p10-12) and juvenile (4-5 weeks) rats using the working heart brainstem preparation that has eupnoeic respiratory drive and preserved cardiorespiratory reflex function (Paton 1996). Neonatal (n=6, 15g) and juvenile (n=23, 70-80g) Wistar rats were deeply anaesthetised with halothane, bisected below the diaphragm and decerebrated. Perfusion was reinstituted through a double lumen catheter in the aorta (allowing perfusion pressure (PP) monitoring). Perfusate flow was adjusted (neonates~7ml/min; juveniles~17ml/min), to obtain a rhythmic incrementing eupnoeic phrenic discharge pattern. Thoracic SNA (T6-10) was recorded using a bipolar suction electrode. All data are mean±SE and unpaired t-test was used to assess significance (P<0.05). The PP in the neonate was lower than in the juvenile (22±1 vs 63±2mmHg, P<0.001). The pattern of tSNA was similar at both ages with prominent respiratory modulation, bursts peaking during the inspiratory and post-inspiratory periods. Power spectral analysis showed the frequency distribution of tSNA was similar to that seen previously in juvenile rats, with a dominant peak at the respiratory frequency. Baroreflex challenges (phenylephrine, 5μg, i.a.) evoked equivalent bradycardia but the sympathetic baroreflex gain was increased in the neonates (Table 1). Activation of the peripheral chemoreflex (NaCN, 100μl, 0.05%, i.a.) evoked similar increases in respiratory frequency, SNA and PP but a smaller bradycardia was found in the neonates. Engagement of the diving response (10°C saline to the snout) evoked equivalent degrees of sympathoexcitation in both groups. Thus we have shown that rats as young as 10 days postnatal have respiratory modulated sympathetic activity that is functionally regulated by peripheral somatic and cardiorespiratory afferents in a manner similar to that seen in more mature animals. In particular our findings extend previous work on the maturation of the heart rate baroreflex in neonates (1, 2) and indicate that the gain of the sympathetic limb is increased.