The presence of electrotonic and dye coupling in region CA1 of the guinea-pig hippocampus was investigated in the in vitro hippocampal slice preparation. No electronic coupling potentials were observed in simultaneous recordings from 101 pairs of pyramidal cells. Also, no electrotonically-coupled short latency depolarizations were observed in more than 75 pyramidal cells in response to antidromic activation of the pyramidal cell population, either in normal bathing medium or in medium with lowered Ca2+ concentration and added Mn2+. When the fluorescent dye Lucifer Yellow was injected into pyramidal cell somas, spread of the dye to other cells (dye coupling) was often observed. Injection of Lucifer Yellow into the dendrites of these neurons resulted in many fewer cases of dye coupling. The failure to find electrophysiological evidence of electrotonic coupling among CA1 pyramidal cells suggests that such coupling is not a functionally important feature of this area of the CNS. The lack of electrophysiological evidence of coupling combined with the observation that the site of Lucifer Yellow injection influences the extent of dye coupling further suggests that at least part of the observed dye coupling may be artifactual. Electrotonic coupling may exist in a small percentage of hippocampal pyramidal cells. However, it is not clear that this small amount of coupling is either necessary or sufficient for the synchronization of neural activity as has been hypothesized to occur during epileptogenesis.