Differences in the locomotor stimulating and rewarding properties of drugs of abuse have been described in several inbred strains of mice, and comparisons of inbred strains with differing responses to drugs of abuse may provide crucial insight into the question of individual vulnerability to the effects of drugs of abuse. The present study was designed to examine the rewarding and locomotorstimulating effects of heroin in C57BL/6J and 129P3/J mice. Heroin produced a robust dose dependent locomotor stimulation in both strains. Both strains also developed conditioned place preference to heroin, again in a dose dependent manner. However C57BL/6J mice developed conditioned place preference to only the two lowest doses of heroin tested, while the 129P3/J counterparts showed conditioned place preference to only the three highest doses tested. These studies indicate that 129P3/J mice are less sensitive to the rewarding effects of heroin than are agematched C57BL/6J mice.