Anterior pituitary cells express -amino butyric acid (GABA)-A receptor-channels, but their structure, distribution within the secretory cell types, and nature of actions have not been clarified. Here we addressed these questions using cultured anterior pituitary cells from postpubertal female rats and immortalized T3-1 and GH3 cells. Our results show that mRNAs for all GABAA receptor subunits are expressed in pituitary cells and that 1/1 subunit proteins are present in all secretory cells. In voltage-clamped gramicidin-perforated cells, GABA induced dose-dependent increases in current amplitude that were inhibited by bicuculline and picrotoxin and facilitated by diazepam and zolpidem in a concentration-dependent manner. In intact cells, GABA and the GABAA receptor agonist muscimol caused a rapid and transient increase in intracellular calcium, whereas the GABAB receptor agonist baclofen was ineffective, suggesting that chloride-mediated depolarization activates voltage-gated calcium channels. Consistent with this finding, RT-PCR analysis indicated high expression of NKCC1, but not KCC2 cation/chloride transporter mRNAs in pituitary cells. Furthermore, the GABAA channel reversal potential for chloride ions was positive to the baseline membrane potential in most cells and the activation of ion channels by GABA resulted in depolarization of cells and modulation of spontaneous electrical activity. These results indicate that secretory pituitary cells express functional GABAA receptor-channels that are depolarizing.