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1. Composite excitatory post-synaptic potentials (e.p.s.p.s) resulting from electrotonic and chemical synaptic junctions were recorded from eighteen interneurones following stimulation of the I2 burster axon in the isolated lamprey spinal cord. 2. In each cell, the half-width of the electrotonic e.p.s.p. was measured and used, together with the cable parameters estimated for the same neurone, to locate the position of synaptic contact made by the I2 axon on the dendrites of the interneurone. The synaptic location ranged from 0.05 to 1.35 space constants with a mean of 0.46. 3. The synaptic potential was simulated using the Rall model of the neurone. When compared with the experimentally recorded e.p.s.p. with the same half-width, the rise-time of the simulated synaptic potential was found to be faster. By changing the value of synaptic distance and/or synaptic current duration the half-width, rise-time, and decay of the simulated synaptic potential fit closely the experimental e.p.s.p. The range of synaptic distance estimated from the simulation decreased considerably (0.2--0.7 space constants; mean 0.52). 4. Direct comparison of synaptic location estimated from histological tracings of dendritic trees from these same cells injected with horseradish peroxidase compared favourably with synaptic location estimated from the simulations. 5. These results support the hypothesis that functionally similar presynaptic axons make synaptic connexions at the same electrotonic distance from the soma on functionally similar post-synaptic cells. This occurs in the face of large variations in physical distance for these same synaptic contacts.