The neurosteroids, pregnenolone sulfate (PREG-S) and tetrahydroprogesterone (TH-PROG), act on the GABAA receptor with antagonist or agonist-like properties, respectively. In this study the effect of the infusion of PREG-S and TH-PROG into the nucleus basalis magnocellularis (NBM) of the rat was examined in a two-trial memory task. The results show that PREG-S (5 ng in 0.5 microliter) enhances memory performance when injected after an acquisition trial; conversely TH-PROG (2 ng in 0.5 microliter) disrupts performance when injected before an acquisition trial. A role for neurosteroids in memory processes subserved by the nucleus basalis magnocellularis is of interest in view of the implication of this structure and these substances in neurodegenerative processes.