Neonatal convulsions are preferably treated with intravenous phenobarbital that contains propylene glycol (PG) as solvent. Very high concentrations of brain PG have been observed with quantitative MRS, especially when low-concentrated phenobarbital medication was used. PG can have serious adverse effects, and the half-life is longer in neonates than in adults. Based on given medication and the interval until MRS examination we estimated a PG half-life in neonatal brain that is at least 30 hours and maybe up to 43 hours. This shows that extremely high and potentially toxic PG concentrations will persist longer than expected in the neonatal brain.