Social isolation of rats for 30 days immediately after weaning results in marked decreases in the cerebrocortical and plasma concentrations of pregnenolone, progesterone, 3α-hydroxy-5α-pregnan-20-one (3α,5α-TH PROG), and 3α,5α-tetrahydrodeoxycorticosterone (3α,5α-TH DOC), as well as a moderate increase in the plasma concentration of corticosterone. This mildly stressful condition has now been shown to increase the sensitivity of rats to the effect of acute ethanol administration on the cerebrocortical and plasma concentrations of neuroactive steroids. The percentage increases in the brain and plasma concentrations of pregnenolone, progesterone, 3α,5α-TH PROG, and 3α,5α-TH DOC, apparent 20 min after a single intraperitoneal injection of ethanol (1 g/kg), were thus markedly greater in isolated rats than in group-housed animals. A subcutaneous injection of isoniazid (300 mg/kg) also induced greater percentage increases in the concentrations of these steroids in isolated rats than in group-housed animals. These results suggest that mild chronic stress, such as that induced by social isolation, enhances the steroidogenic effect of ethanol, a drug abused by humans under stress or affected by neuropsychiatric disorders. Social isolation also induced hyper-responsiveness of the hypothalamic–pituitary–adrenal (HPA) axis, as was apparent after reduction of GABA-mediated inhibitory tone by isoniazid administration.