1. The influence of dexamethasone on morphine analgesia in three different strains of mice (Swiss, DBA/2J and C57BL/6) was studied by using the tail flick test. 2. I.c.v. as well as i.p. injections of dexamethasone did not modify nociceptive response in all strains. 3. I.c.v. injection of dexamethasone significantly reduced morphine analgesia in Swiss mice whereas no effects were observed in DBA/2J and C57BL/6 mice. 4. In addition, i.p. injection of dexamethasone significantly reduced morphine analgesia in all three strains. 5. These results suggest that the use of different genetic strains may provide an useful approach for studying dexamethasone-morphine analgesia interaction.