The effects of 17 beta-estradiol and progesterone on the uptake of adenosine by rat cerebral cortical synaptosomes have been correlated with the ability of these gonadal steroids to potentiate the depressant actions of adenosine on the spontaneous firing of rat cerebral cortical neurons. 17 beta-estradiol hemisuccinate competitively inhibited adenosine uptake with a Ki of 0.5 microM (Lineweaver-Burk plot) or 0.78 microM (Dixon plot). The Ki for progesterone was 0.34 microM (Lineweaver-Burk plot) or 0.36 microM (Dixon plot). When applied by iontophoresis, both steroids potentiated the depressant effects of adenosine on the firing of rat cerebral cortical neurons. Potentiation of the effect of endogenously released adenosine would account for the central depressant actions of these steroids.