Male C57BL/6 and DBA/2 mice were injected intrahippocampally with either naloxone (0.5 microgram) or morphine (1.0 microgram), or saline vehicle alone and, after 15 min, some 12 behavioural components carried out in a novel environment were recorded for 20 min. Naloxone reduced exploratory rearing responses, wall-leaning and object-sniffing in strain C57BL/6 and augmented these behaviours in strain DBA/2, while morphine depressed the scores in both. In conjunction with previously obtained evidence that the mouse hippocampus contains a genotype-dependent cholinergic mechanism which regulates responses to novelty, these findings support the hypothesis that hippocampal opioid peptides modulate the cholinergic control of exploration in mice, possibly indirectly through GABAergic pathways. In contrast, locomotor activity, defaecation and tail elevation remained practically unaffected by the two drugs, and grooming showed another kind of genotype-treatment interaction, that is to say, after morphine.