1. The effects of the naturally occurring neurosteroid tetrahydrodeoxycorticosterone (THDOC) on GABAA receptor-mediated miniature, spontaneous and evoked IPSCs was tested using patch-clamp techniques in slices of hippocampus and cerebellum from rats at two developmental stages ( approximately 10 and approximately 20 days postnatal). The cells studied were hippocampal granule cells and cerebellar Purkinje and granule cells. 2. Most miniature GABAergic currents (mIPSCs) decayed with two exponentials and neurosteroids caused a approximately 4-fold increase in the decay time constant of the second exponential at the highest concentration used (2 microM). Similar effects were seen at high concentrations of THDOC (1-2 microM) in all cell groups tested. No effects were seen on amplitude or rise time of mIPSCs. 3. The effects of THDOC (1 microM) were shown to be stereoselective and rapidly reversible, indicating that the neurosteroid binds to the GABAA receptor, rather than acting genomically. 4. At concentrations of THDOC likely to occur physiologically (50-100 nM), the decay time of IPSCs was also enhanced (25-50 %) in all cerebellar cell groups tested. In contrast, at 100 nM THDOC, seven of 11 hippocampal granule cells were sensitive from the 10 day group but the 20 day hippocampal granule cells showed no significant enhancement in the presence of these lower concentrations of THDOC. 5. The differences in sensitivity of hippocampal and cerebellar cells to THDOC are compared to data reported in the literature on regional development of expression of different receptor subunits in the brain and it is suggested that the progressive relative insensitivity of the 20 day hippocampal cells may depend on increasing expression of the delta subunit of the GABAA receptor and possibly an increase in the alpha4 subunit.