Antidromic stimulation in slices of rat hippocampal cortex was used to test for electrotonic coupling between CA1 pyramidal neurons. The slices were incubated in low Ca2+ medium containing Mn2+, in order to block chemical synapses. In a small percentage of intracellular recordings, short-latency depolarizations could be observed after chemical transmission had been blocked. Preceding somatic action potentials did not block the depolarizations. This indirect test provides electrophysiological evidence for coupling between some CA1 pyramidal cells.