The time of weaning has numerous effects on neurobehavioral development. Previous findings suggest that the early weaning influences development of aggressive behaviors. Behavioral and neuroendocrine responsiveness to stressors in the adulthood are also influenced by maternal care received early in life, and early-weaned male mice and rats show higher responsiveness to acute stresses than do normally weaned males. Therefore, it is conceivable that early weaning influences stress-related aggressive behaviors. We investigated the effects of early weaning on aggressive behaviors under two stress conditions: social stress (social instigation) and ecological stress (food restriction), both of which augment aggression. Male ICR mice were divided into two groups based on weaning period. Normally weaned mice (weaned PD21) showed twice the baseline level of attack bites after 5 min of social instigation, whereas early-weaned animals (weaned PD14) were not more aggressive following social instigation. However, the early-weaned mice were more aggressive after food restriction stress than were the normally weaned mice, suggesting lower threshold for aggressive behavior after food shortage. We also measured 5HT1A and 5HT1B receptor mRNA expression in the hippocampus which involved in aggression using real-time PCR. Early-weaned mice had lower 5HT1B expression levels than did normally weaned mice; no effect was found for 5HT1A expression. These results suggest that manipulation of weaning time modulates adult aggressive behavior depending on the stressors imposed and that this change may involve the 5HT1B receptor system in the hippocampus.