Many animals exhibit pronounced changes in physiology and behavior on a seasonal basis, and these adaptations have evolved to promote survival and reproductive fitness among conspecifics. While the neuroendocrine pathways mediating seasonal reproduction are generally well-studied, far less is known about the mechanisms underlying seasonal changes in social behavior, particularly outside of the context of the breeding season. Previous work from our lab suggests that seasonal shifts in secretion of the pineal hormone melatonin (MEL) are important in regulating territorial aggression in Siberian hamsters (Phodopus sungorus); it is unclear, however, how MEL signaling at the level of the receptor modulates this behavior. In this study, we infused a MEL 1a receptor (Mel1aR)-overexpressing or control lentivirus bilaterally into the adrenal glands of male Siberian hamsters. Animals were then housed in long-day (LD) or short-day (SD) photoperiods (characteristic of the breeding and non-breeding seasons, respectively) and administered timed MEL or control injections, and aggressive behavior was quantified following 10 weeks of treatment. LD males infused with Mel1aR-overexpressing lentivirus had significantly higher adrenal Mel1aR expression than LD control males, as determined via quantitative PCR. Furthermore, adrenal Mel1aR overexpression and timed MEL injections differentially affected body and relative reproductive mass in LD males. The potential relationship between adrenal Mel1aR expression and aggression will also be examined. Collectively, our findings provide insight into how MEL signaling via Mel1aR mediates aggressive behavior in seasonally breeding species.