Groups of eight rats each were preexposed on four occasions to 10 or 20 mg/kg morphine sulfate, IP, either in activity boxes where activity was measured for two hours (COND, conditioning groups) or in their home cages (UNPAIRED groups). On alternate days these groups were administered saline in the other environment. Two groups of eight rats each served as CONTROL groups (one for each preexposure dose) and were administered saline in both environments. On the day following morphine preexposure, all animals were administered 0.5 mg/kg d-amphetamine sulfate, IP, prior to being tested in the activity boxes. On this test, the COND group preexposed to 10 mg/kg morphine showed higher levels of activity than either of its respective UNPAIRED or CONTROL groups. The COND group preexposed to 20 mg/kg morphine was significantly more active than its unpaired group, but not more active than its CONTROL group. The implications of such environment-specific cross-sensitization between the activity effects of opiate and stimulant drugs are discussed.