Dopamine was found to have two electrophysiological effects on CA1 pyramidal cells in rat hippocampal slices. It increased the slow afterhyperpolarisation caused by a slow Ca2+-activated K+ conductance and it had an effect on action potentials that is postulated to be due to an increase in a fast Ca2+-activated K+ conductance. A given CA1 cell showed either one or both of the two responses to dopamine, or no response.