PHYSIOLOGICAL properties of mouse mossy cells were analyzed in slice preparations and compared with those of mouse CA3 pyramidal cells. They had larger input resistances, less spike-frequency adaptation, more anomalous rectification, larger amplitude spontaneous EPSP and higher frequency of spontaneous EPSP than CA3 pyramidal cells. In these respects, they resembled rat mossy cells. However, in contrast to rat mossy cells, mouse mossy cells showed pronounced burst after-hyperpolarization, comparable to that of CA3 pyramidal cells. This study revealed that mouse mossy cells are distinct from CA3 pyramidal cells in their physiological properties and further suggested that they might be less vulnerable to excitotoxic damage than rat mossy cells, as the burst after-hyperpolarization is supposed to be important in maintaining a neuron in a relatively nonexcited state.