The aim of our study was to investigate the influence of gonadal steroids on the expression of different GAD isoforms. Here we show that, in addition to the adult GAD forms, the two embryonic splice variants of GAD67 mRNA and the truncated GAD25 are present in the adult rat olfactory bulb, a brain region with high synaptic plasticity, which has preserved some features of the developing brain. By Western blot analysis, we could demonstrate that the expression of the embryonic GAD25 is cyclic in females: its quantity is higher on estrus day. Furthermore, in ovariectomized animals 17-β-estradiol treatment induced an increase of GAD25 within 3 h, reaching a maximum at 9–12 h. Our data are compatible with the interpretation that the embryonic GAD isoforms may play a role in the neuroplastic changes induced by sexual steroids.