Much like the early days of emotion research, study of the connections between biochemistry and emotion has largely examined these processes in intrapersonal contexts. This chapter highlights research on biochemistry and emotion with clear implications for interpersonal emotion dynamics and highlights the importance of considering the interpersonal context. To do so, we focus on two biochemical systems,: the serotonin system and the immune system, especially inflammation. A particular focus is on the effects of acetaminophen, which alters both central serotonin and inflammatory signaling. Based on work in the intrapersonal context, both systems affect reactivity to positive and negative events and both systems affect empathy, a core interpersonal emotional process. Underscoring the importance of moving biochemical emotions research into an interpersonal context, the direction of emotional effects of these systems appears to be context dependent. For example, much work indicates that inflammation decreases positive emotion, but recent work in an interpersonal context suggests that inflammation can increase positive reactivity when interacting with close others. Thus, we see the interpersonal context as a critical factor for understanding the relationship between biochemistry and emotion. Greater attention to the interpersonal context is likely to lead to advances in understanding of relationships, emotions, biochemistry, and health.