The effects of social isolation on behavior, neuroactive steroid concentrations, and GABAA receptor function were investigated in rats. Animals isolated for 30 days immediately after weaning exhibited an anxiety-like behavioral profile in the elevated plus-maze and Vogel conflict tests. This behavior was associated with marked decreases in the cerebrocortical, hippocampal, and plasma concentrations of pregnenolone, progesterone, allopregnanolone, and allotetrahydrodeoxycorticosterone compared with those apparent for group-housed rats ; in contrast, the plasma concentration of corticosterone was increased in the isolated animals. Acute footshock stress induced greater percentage increases in the cortical concentrations of neuroactive steroids in isolated rats than in group-housed rats. Social isolation also reduced brain GABAA receptor function, as evaluated by measuring both GABA-evoked Cl- currents in Xenopus oocytes expressing the rat receptors and tert-[35S]butylbicyclophosphorothionate ([35S]TBPS) binding to rat brain membranes. Whereas the amplitude of GABA-induced Cl- currents did not differ significantly between group-housed and isolated animals, the potentiation of these currents by diazepam was reduced at cortical or hippocampal GABAA receptors from isolated rats compared with that apparent at receptors from group-housed animals. Moreover, the inhibitory effect of ethyl-β-carboline-3-carboxylate, a negative allosteric modulator of GABAA receptors, on these currents was greater at cortical GABAA receptors from socially isolated animals than at those from group-housed rats. Finally, social isolation increased the extent of [35S]TBPS binding to both cortical and hippocampal membranes. The results further suggest a psychological role for neurosteroids and GABAA receptors in the modulation of emotional behavior and mood.