Using the combination of [3H]thymidine autoradiography and immunohistochemistry for the detection of glutamic acid decarboxylase (GAD), we have determined that most of the cells demonstrating GAD-like immunoreactivity in the hippocampal formation of the rat become postmitotic on fetal day 14, though some cease dividing on fetal days 13 and 15-18. GAD-positive cells in all fields of the hippocampal formation appear to differentiate during roughly the same period though there is evidence for both septotemporal and regional variations in the pattern of development.