BACKGROUND: Due to its availability and perceived safety, paracetamol is recommended even during pregnancy and for neonates. It is used frequently alone or in combination with other drugs required for the treatment of various chronic conditions. The aim of this study was to investigate potential effects of drug interactions on paracetamol metabolism and its placental transfer and entry into the developing brain. METHODS: Sprague Dawley rats at postnatal day P4, pregnant embryonic day E19 dams, and non-pregnant adult females were administered paracetamol (15 mg/kg) either as monotherapy or in combination with one of seven other drugs: cimetidine, digoxin, fluvoxamine, lamotrigine, lithium, olanzapine, valproate. Concentrations of parent paracetamol and its metabolites (paracetamol-glucuronide, paracetamol-glutathione, and paracetamol-sulfate) in plasma, cerebrospinal fluid (CSF) and brain were measured by liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) and their entry into the brain, CSF and transfer across the placenta were estimated. RESULTS: In monotherapy, concentration of parent paracetamol in plasma, CSF, and brain remained similar and at all ages brain entry was unrestricted. In combination therapies, CSF entry of paracetamol increased following co-treatment with olanzapine. Placental transfer of parent paracetamol remained unchanged, however, transfer of paracetamol-sulfate increased with lamotrigine co-administration. Acutely administered paracetamol was more extensively metabolized in adults compared to younger ages resulting in increased concentration of its metabolites with age. CONCLUSIONS: Developmental changes in the apparent brain and CSF entry of paracetamol appear to be determined more by its metabolism, rather than by cellular control of its transfer across brain and placental barriers.