Hippocampal pyramidal cells, among the most-studied cortical neurones, display several interesting properties. In particular, intracellular recordings in both in vivo1 and in vitro2 preparations have shown that the pyramidal neurones of the CA3 region of the hippocampus have, in addition to the more conventional single spike, the capacity to generate spontaneous bursts of action potentials (APs). The burst-firing patterns have been well documented, but their underlying electrophysiological mechanisms are not completely understood. The bursts seem to be an intrinsic response of individual neurones and can be triggered by weak orthodromic stimulation or by direct depolarization of the somatic membrane. In molluscan and spinal systems, voltage-clamp techniques have revealed slow, voltage-sensitive, inward currents underlying this type of firing behaviour3-7. The use of these voltage-clamp techniques to study this phenomenon in hippocampal neurones has previously been precluded by the small size of the neurones and by the difficulty of recording from them in intact preparations. However, the development of methodology for voltage-clamping with a single microelectrode (SEC)8 and for the maintenance of hippocampal slices in vitro9 has made it possible for us to examine the membrane currents in these CA3 neurones. We report here a slow inward current in CA3 neurones and suggest that it is carried by calcium ions. © 1980 Nature Publishing Group.