1. The patterns of on-going synaptic events recorded intracellularly in neurones of superior cervical ganglia (SCG)of anaesthetized female rats were analysed by constructing inter-event interval histograms, autocorrelograms, ln-survivor curves and histograms triggered by the arterial pulse wave and by the intercostal EMG. 2. In 11/12 cells with on-going frequencies > 0.5 Hz, one or two inputs were strong (i.e. always suprathreshold). In five cells, action potentials also arose from synaptic potentials with amplitudes close to threshold. 3. Synaptic events in 5/11 neurones tested were phase-related to the arterial pressure wave (i.e. had cardiac rhythmicity, CR). 4. Synaptic events in 9/10 neurones tested (including all with CR) were phase-related to the intercostal EMG and/or their autocorrelograms showed peaks at multiples of the respiratory interval (i.e. had respiratory rhythmicity, RR). 5. The intervals between all synaptic events were exponentially distributed in 8/12 neurones although intervals between single strong events showed peaks related to the respiratory cycle. Bursts occurred only by chance. 6. Event patterns could be simulated by combining events from several respiration-modulated inputs with their timing distributed over nearly half the cycle. From the simulations, the mean number of active preganglionic inputs was estimated to be approximately 6 with mean discharge frequency approximately 0.4 Hz. 7. We conclude that, in the spontaneously breathing anaesthetized rat, most preganglionic neurones to the SCG fire with relatively low probability in relation to the respiratory cycle. Rhythms in a postganglionic neurone reflect the activity of its suprathreshold preganglionic inputs.