To evaluate whether post-training parafascicular intracranial electrical stimulation (PF ICS) can improve acquisition (five sessions, one daily, 10 trials each) and long-term retention (LTR) (one session, 10 trials) of two- way active avoidance conditioning in rats, experimental subjects (Ss) were implanted with an electrode at the parafascicular nucleus (PF). Control Ss were sham operated without implanted electrodes. Immediately after each acquisition session Ss in the experimental groups were stimulated in the PF during 2- or 10-min periods. After histological analyses Ss were grouped according to the antero-posterior PF stereotaxic coordinates for the electrode tip locations. Ss stimulated at the posterior PF area improved acquisition and LTR of the conditioning, whereas Ss stimulated at the central PF area showed detrimental effects upon conditioning. Electrical stimulation of the anterior PF area did not affect conditioning performance. We conclude that PF appears to have differential modulatory roles in acquisition and retention of two-way active avoidance depending on the PF area involved. © 1995 Academic Press Inc.