The effects of the selective opiate kappa-receptor agonist tifluadom on memory were investigated in a passive avoidance task in 3 sets of experiments carried out with DBA/2 (DBA) mice both familiarized and unfamiliarized with the apparatus. In a first set of experiments, tifluadom (1.0 or 2.5, but not 0.5 mg/kg) administration immediately after training impaired retention performance of non-familiarized mice. This impairment was still evident when the drug was injected 15 or 30, but not 60 min after training. A second set of experiments was carried out with mice familiarized with the apparatus. Tifluadom was less effective in impairing memory in this group of animals, as compared with non-familiarized mice. Finally, in a third set of experiments, carried out with non-familiarized mice, a 15 min immobilization stress, which was ineffective when administered alone, enhanced the effects of tifluadom (1.0 mg/kg). The results are discussed in terms of attenuation of emotionality, resulting in impaired retention, following post-training opiate administration.