1. Re-innervation of partially denervated sympathetic ganglion cells was investigated using intracellular recording in guinea-pig lumbar paravertebral ganglia in vitro. The question addressed was whether the pattern of innervation by strong (suprathreshold) and weak (subthreshold) inputs seen normally was restored during re-innervation. 2. L5 ganglion cells each normally received 3.9 +/- 0.2 preganglionic inputs of which 1.2 +/- 0.1 were strong. Only 0.9 +/- 0.1 inputs arose from the L4 segment, the last of the thoracolumbar outflow, and only 11 % of these were strong. 3. Three to five weeks after cutting the sympathetic chain above the L4 white ramus, each neurone received 2.1 +/- 0.1 inputs after sprouting of surviving axons. Nearly 60 % of neurones received a strong input and the normal ratio of weakstrong synapses was restored. 4. Total charge transfer evoked by L4 inputs increased from 8.8 +/- 1.4 to 27. 6 +/- 2.4 pC per neurone after re-innervation, reaching 77 % of that in normal ganglia. This was primarily due to the formation of new strong inputs of normal size. 5. The synaptic events at the new strong synapses and the types of Ca2+ channel mediating transmitter release (N-type and channels resistant to specific antagonists) were the same as those at control strong synapses. 6. The data indicate that, following partial denervation, sprouting of surviving preganglionic axons results in the preferential formation of strong synapses with the same characteristics as those in normal ganglia. Thus the pattern of functional transmission by a single strong input to each cell was restored rather than the recovery of the number of synaptic connections.