Prostaglandin F2α-d4 (PGF2α-d4) contains four deuterium atoms at the 3, 3', 4, and 4' positions. It is intended for use as an internal standard for the quantification of PGF2α by GC- or LC-mass spectrometry. PGF2α is a widely distributed prostaglandin occurring in many species.{187,421,796} It causes contraction of vascular, bronchial, intestinal, and myometrial smooth muscle, and also exerts potent luteolytic activity.{187} PGF2α exhibits its receptor mediated physiological activity at 50-100 nM.{187} Maximal ovine myometrial contraction can be achieved at 125 nM PGF2α in vitro.{1651}