Clinical epileptologists since Gowers have known that hormones may affect central nervous system excitability. Progesterone has been noted to have anticonvulsant properties in both clinical and animal studies. The effect of progesterone on central nervous excitability in the immature animal, however, has not been studied. This study evaluated the effects of progesterone on the rate of kindling in adult and immature animals. While progesterone has no effect on kindling in the adult animal, in the immature animal progesterone markedly inhibited kindling, preventing generalization of seizures. This study demonstrates that progesterone has a marked age-dependent effect on neuronal excitability.