Latanoprost (free acid)-d (Lat-FA-d) 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 Lat-FA by GC- or LC-mass spectrometry. Latanoprost is an F-series prostaglandin (PG) analog which has been approved for use as an ocular hypotensive drug. It is the isopropyl ester of a PGF analog containing an aromatic group (17-phenyl) in the ω-chain. As an isopropyl ester, latanoprost acts as a prodrug which is converted to Lat-FA by endogenous esterase enzymes. Lat-FA is a potent FP receptor agonist with an EC value of 3.6 nM for human FP receptors, which is twice the potency of PGF and more than 200 times more potent latanoprost. The efficacy of PG analog esters for the treatment of glaucoma correlates closely with the FP receptor binding affinity of the free acid. However, Lat-FA is more irritating and less effective than the prodrug latanoprost when applied directly to the eyes of human glaucoma patients.