The forebrains of mammals and birds appear quite different in their gross morphology, making it difficult to identify homologies between them and to assess how far they have diverged in organization. Nevertheless one set of forebrain structures, the basal ganglia, has been successfully compared in mammals and birds. Anatomical, histochemical, and molecular data have identified the avian homologues of the mammalian basal ganglia and indicate that they are very similar in organization, suggesting that they perform similar functions in the two classes. However, the physiological properties of the avian basal ganglia have not been studied, and these properties are critical for inferring functional similarity. We have used a zebra finch brain slice preparation to characterize the intrinsic physiological properties of neurons in the avian basal ganglia, particularly in the input structure of the basal ganglia, the striatum. We found that avian striatum contains a cell type that closely resembles the medium spiny neuron, the principal cell type of mammalian striatum. Avian striatum also contains a rare cell type that is very similar to an interneuron class found in mammalian striatum, the low-threshold spike cell. On the other hand, we found an aspiny, fast-firing cell type in avian striatum that is distinct from all known classes of mammalian striatal neuron. These neurons usually fired spontaneously at 10 Hz or more and were capable of sustained firing at very high rates when injected with depolarizing current. The existence of this cell type represents an important difference between avian striatum and mammalian dorsal striatum. Our data support the general idea that the organization and functional properties of the basal ganglia have been largely conserved in mammals and birds, but they imply that avian striatum is not identical to mammalian dorsal striatum.