Polyspecific organic cation (OC) transporters play important roles in the disposition of clinically used drugs, including drugs used during pregnancy. Pregnancy is known to alter the expression of drug-metabolizing enzymes and transporters, but its specific effect on OC transporters has not been well defined. Using quantitative polymerase chain reaction and liquid chromatography coupled with tandem mass spectrometry targeted proteomics, we determined the effect of pregnancy and gestational age on mRNA and protein expression of major OC transporters in the kidney, liver, and placenta in mice with timed pregnancies. Human organic cation transporter 3 (hOCT3) expression was further investigated in human placentas from the first and second trimesters and at term. Our results showed that pregnancy had a marginal effect on renal mouse organic cation transporter 1/2 (mOct1/2) expression but significantly reduced mouse multidrug and toxin extrusion transporter 1 (mMate1) expression by 20%-40%. Hepatic expression of mOct1 and mMate1 was minimally affected by pregnancy. Human and mouse placentas predominantly expressed OCT3 with little expression of OCT1/2, MATE1/2, and plasma membrane monoamine transporter (PMAT). The hOCT3 protein in first and second trimester and term placentas was quantified to be 0.23 ± 0.033, 0.38 ± 0.072, and 0.36 ± 0.099 fmol/μg membrane protein, respectively. In contrast with the moderate increase in hOCT3 protein during human pregnancy, mOct3 expression in the mouse placenta was highly dependent on gestational age. Compared with gestational day (gd) 10, placental mOct3 mRNA increased by 37-fold and 46-fold at gd 15 and 19, leading to a 56-fold and 128-fold increase in mOct3 protein, respectively. Our study provides new insights into the effect of pregnancy on the expression of polyspecific OC transporters and supports an important role of OCT3 in OC transport at the placental barrier.