To evaluate a possible role of the parafascicular nucleus (PF) in modulating distributed two-way active avoidance acquisition and long-term retention (LTR), we designed two experiments. Experiment I was aimed at checking whether posttraining PF intracranial electrical stimulation (PF ICS) can improve the acquisition and/or the LTR of the task. All subjects (Ss, male Wistar rats) were implanted with an electrode at the PF. After each learning session two groups of Ss were stimulated for 10 (ICS-10 group) and 5 (ICS-5 group) min, respectively. A Control group never received PF ICS, while Ss in an ICS-Control group received PF ICS only during a previous search for a nonconvulsive current intensity. Unexpectedly, the ICS-Control group showed poor performance of the task compared to the remaining groups. Since the histological analyses showed that the pretraining ICS treatment produced some PF tissue lesion, Experiment II was aimed at evaluating the effects upon the same task of (1) pretraining PF electrolytical lesions (PF-Lesion group) and (2) posttraining PF ICS treatment (ICS group) at a lower current intensity and without a previous search for nonconvulsive current intensity. PF pretraining lesion decreased conditioning, while posttraining PF ICS did not affect it. We concluded that PF could have a modulatory role in acquisition, and might also contribute to posttraining consolidation, of a distributed two-way active avoidance. © 1995 Academic Press. All rights reserved.