The immunostimulant lipopolysaccharide (LPS) produces numerous physiological and behavioral effects in adult rodents. The present study examined the effects of this endotoxin on activity levels and weight gain in 20-day-old preweanling rats. Experiment 1 demonstrated that 400 ~g/kg of LPS produced a significant decline in locomotor activity 1 hr postinjection, and both 200 ~g/kg and 400 ~g/kg significantly suppressed weight gain over the first 24 hr posttreatment. The effect on weight gain dissipated by 48 hr following LPS administration. Experiment 2 replicated the effects of 400 ~g/kg LPS on activity at 1 hr and on weight gain, and also assessed its effects upon activity level 24 hr following administration. Despite the significant decline in weight gain over the initial postinjection day, LPS animals' activity level was normal 24 hr after injection. These results indicate that the endotoxin LPS produces significant but transient decreases in both locomotor activity and weight gain in preweanling rats. The relationship of these findings to other issues involving physiology and sickness behavior is discussed.