Dermorphin and [D-Ala2-D-Leu5]enkephalin, administered intracerebroventricularly (icv) immediately after training in a passive avoidance test, exerted dose- and strain-dependent effects in DBA/2 (DBA) and C57BL/6 (C57) mice. In fact, the peptides impaired, at a low dose (5 ng), the retention performances of both strains; a higher dose (50 ng) impaired retention in DBA but improved it in C57 mice. A dose of naloxone (0.3 microgram, icv), which was ineffective when administered alone, antagonized the effects observed. The results are discussed in terms of strain differences in central mechanisms.