This document summarizes information about serotonin and histamine, including their biosynthesis, pharmacological effects, mechanisms of action, and examples of agonists and antagonists. Serotonin is produced in the gastrointestinal tract and central nervous system. It has diverse effects, including acting as a neurotransmitter, regulating cardiovascular and gastrointestinal functions, and mediating platelet aggregation. Histamine acts through H1, H2, and H3 receptors and is involved in allergic responses. Examples are given of several drugs that act as serotonin or histamine receptor agonists or antagonists for conditions like migraine, depression, anxiety, motion sickness, and allergy treatment.