Trazodone is used to treat anxiety disorder, insomnia, and sleep disorders, which occur in ∼15% of pregnant and lactating women. However, pharmacokinetic information on the transfer of trazodone and its active metabolite, 1–chlorophenylpiperazine (mCPP), across the placenta or into breast milk is limited. In this study, we describe the pharmacokinetic profile of trazodone and mCPP concentrations in maternal and neonatal blood and breast milk. A 44-year-old female received oral trazodone 50 mg once daily during pregnancy (28-38 gestational weeks) and lactation, along with etizolam for anxiety disorder with depressive syndrome. A male infant weighing 2,918 g was born at 38 weeks of gestation. Because of persistent respiratory disturbance, oxygenation was initiated immediately after birth, and the infant was admitted in the neonatal intensive care unit for 5 days. No pulmonary dysfunction or birth defects were detected, and no medication and circulatory support were needed during admission. Trazodone and mCPP concentrations in cord blood at 7.4 hours after maternal dosing were 267.6 and 22.8 ng/mL, respectively, which were comparable with maternal serum levels. The trazodone and mCPP concentrations in breast milk collected 7.2 hours after maternal dosing were 50.2 and 3.2 ng/mL, respectively. The infant developed normally, with no drug-related adverse effects at the 1-, 3-, and 6-month postpartum checkups. Trazodone and its active metabolite were transferred into placenta and breast milk. However, their effects in utero could not be clarified. Further studies are warranted to assess the safety of trazodone in fetuses and breastfed infants.