Neurogenesis of GABAergic neurons in the rat area dentata was studied combining [3H]thymidine autoradiography with immunostaining for glutamate decarboxylase (GAD), the gamma-aminobutyric acid (GABA) synthesizing enzyme. GAD immunocytochemistry stained many neurons located mainly in the hilar region but also in the granular and molecular layers. Granule cells were not immunoreactive. On embryonic day (E) 14, [3H]thymidine injection labelled 12.8% of GAD-positive (+) neurons in sections of the area dentata processed at an age of 40 days postnatally. This proportion decreased to 1.8% on E17 and to 1% on E18. No GAD (+) neurons were labelled by thymidine injection on E19, while non-immunoreactive granule cells and CA4 pyramids were still labelled, indicating persisting neurogenesis of those cells.