Dopamine-d4 is intended for use as an internal standard for the quantification of dopamine by GC- or LC-MS. Dopamine is an endogenous catecholamine neurotransmitter synthesized from the amino acid L-tyrosine that acts as an agonist at dopamine receptors (D1-5). Dopamine is mainly synthesized in the substantia nigra and ventral tegmental area, and is a precursor in norepinephrine and epinephrine biosynthesis. Dopamine-containing neurons in the brain are involved in reward-motivated behavior, motor control, and hormone release. Dopamine is also synthesized in the adrenal glands where it exerts peripheral paracrine functions including control of vasodilation, sodium excretion, insulin production, gastrointestinal motility, and the activity of lymphocytes. Loss or damage of dopaminergic neurons in the substancia nigra is associated with Parkinson’s disease.