Abstract: The locomotor achvities (LA) of rats were tested every 15 mins for 95 mins after morphine injected intraperitoneally with 8, 4, 2, 1mg/kg/day respectively once a day during the 8-day experiment,and the number of squares (10 X 10 cm2) which the rats passed by in 5 mins were recorded. Results showed that there were significant differences between the groups treared with 8 or 4mg/ kg / day of morphine and normal saline on day2-8, and between the groups with 2mg/kg/day of morphine and the control group on day2, 5 and 6. Thus 4mg / kg / day was the most suitable low dose of morphine for activating LA. The peak effect of 4mg/kg/day of morphine was at 15-20 mins after injection, and it kept going up for 8 days. It was suggested that low dose of morphine may be more stutable for related behavioral effects such as those of conditioned place preference or self-administration, and the best time course for training tasks may be within 20 min after the rats were exposed to morphine.

Key words: rats, low dose morphine, locomotor activity