Although it is known that selective serotonin reuptake inhibitors (SSRIs), as other antidepressants, elevate mood only after 3-4 weeks of treatment, the mechanism responsible for this delay is not understood. SSRIs have been demonstrated to alter the levels of neurosteroids such as allopregnanolone (THP) which possess anxiolytic and mood-elevating properties. We compared the effect of 9 and 21 days i.p. administration of paroxetine, a potent SSRI, on the synthesis of THP and its precursor, 5alpha-dihydroprogesterone (DHP), in the mouse cortex, hypothalamus and olfactory bulb. Cortex, olfactory bulb and hypothalamus synthesized levels of DHP were significantly raised after 9 days of paroxetine administration, whereas a significant rise in the THP synthesized level was observed only after 21 days of treatment. Peripheral synthesis of DHP, measured by the level in serum, significantly increased after 9 days, but reverted to normal values after 21 days. No increase was detected in serum THP levels either after 9 or 21 days treatment. Differences in peripheral and brain synthesis indicates independence in brain synthesis. The data indicate that paroxetine administration differentially increases [3H]DHP and [3H]THP content, depending on the duration of the treatment. Our results suggest that brain THP may be involved in the antidepressive and anxiolytic activity of paroxetine.