Infusion of 5-HTP to Holstein and Jersey dairy cows during the transition period increases serotonin content, alters the profile of the 5HTR2 family of receptors, and elevates markers of cellular proliferation and cell turnover in the liver on d1 and d7 post-partum. Specifically, Holstein cows infused with 5-HTP have increased 5HTR2A, 5HTR2B, and PCNA mRNA abundance, as well as increased numbers of CASP3 and Ki67-positive cells when compared to control cows. In addition, H 5-HTP cows have increased PC, but not PEPCK2, mRNA expression compared to H CON cows. Importantly, these changes are accompanied by only minor effects (reduced milk yield on d1 in J 5-HTP versus J CON cows, for example) on a variety of health-related parameters detectable in the circulation. In fact, our **data** reflects a trend towards decreased BHBA concentrations during a key window at 6 to 10 days post-partum in Jersey cows, with the important caveat that circulating calcium concentrations were also decreased in this given time window in Jersey cows. Given that the effects of serotonin are heavily dependent on dose and on timing, further studies should examine the comparative benefits of administering 5-HTP at a higher dose and potentially further into the lactation. Pre-partum infusions of 1.0 mg/kg 5-HTP alter local, hepatic serotonin dynamics towards increased cell proliferation necessary to meet the demands of lactation primarily in Holstein cows, with no adverse effects at the level of the circulation or adverse health outcomes. 