Oxytocin causes contraction of the smooth muscle of the uterus and of the mammary gland. Acts by binding to oxytocin receptor (OXTR) (By similarity). Interacts with oxytocin receptor (Ki=1.5 nM) (By similarity). Interacts with vasopressin V1aR/AVPR1A (Ki=37 nM), V1bR/AVPR1B (Ki=222 nM), and V2R/AVPR2 receptors (Ki=823 nM) (By similarity). Belongs to the vasopressin/oxytocin family.