1. In twenty-five sodium pentobarbitone (alpha-chloralose supplemented)-anaesthetized, artificially ventilated and paralysed rats, postganglionic sympathetic single unit activity was recorded at the level of the adventitia of the caudal ventral artery of the tail using a focal recording technique. 2. Ten units were identified as being sympathetic in nature, as they were activated following electrical stimulation of the lumbar sympathetic chain. The on-going activity of seven of these was blocked by hexamethonium (6-12 mg kg-1). 3. The units were not under tonic baroreceptor modulation, as indicated by the lack of pulse modulation of discharge. Respiratory modulation was apparent, with neurones firing mainly during expiration (phrenic silence), and activity was influenced also by the lung inflation cycle. Whole-body warming decreased unit activity. 4. Interspike interval and autocorrelation analysis showed that unit discharge was dominated by the respiratory rhythm and that units tended to discharge in bursts (often duplets). It is suggested that the intraburst interval may be determined by a hypothetical sympathetic oscillator. 5. This study presents the first analysis of single unit activity recorded in vivo from sympathetic fibres innervating an identified blood vessel.