Serotonin in the mammary gland is known to regulate processes such as calcium homeostasis, tight junction permeability, and milk protein gene expression. The objective of this study was to discover novel genes, pathways and functions which serotonin modulates during lactation. The rate-limiting enzyme in the synthesis of non-neuronal serotonin is tryptophan-hydroxylase (TPH1). Therefore, we used TPH1 knock-out mice dams (serotonin deficient) and compared them to wild-type dams and also Tph1 deficient dams injected daily with 5-HTP. Mammary gland tissues were collected on day 10 of lactation and then analyzed by RNA sequencing. Overall design: Genome-wide gene expression profiles of 12 mouse mammary gland samples were evaluated using RNA sequencing; these 12 samples belong to wild-type dams (WT; n = 4), Tryptophan hydroxylase (Tph1) knock-out dams (KO; Tph1 deficient; n = 4), and Tph1 deficient dams injected daily with 5-HTP (RC; n = 4). Mammary tissues were collected on day 10 of lactation and then underwent RNA extraction, library generation, and subsequent sequencing.