Abstract High performance liquid chromatography with electrochemical detection (HPLC-EC) was adapted for the analysis of metabolites of monoamines (MA) in the developing brain of the rat. 3-Methoxy-4-hydroxy-phenyl-ethylene glycol (MOPEG), dihydroxy phenylacetic acid (DOPAC), homovanillic arid (HVA) and 5-hydroxy-3-indoleacetic acid (5-HIAA) were found in significant amounts in forebrain of the foetal rat from gestational day (GD) 17. The developmental pattern of the four metabolites was essentially similar in male and female animals. Except for transient fluctuations at around birth, concentrations of metabolites increased gradually throughout late gestation and early postnatal life. In late gestation and at birth the absolute amounts were small, while the concentrations were comparable to adult levels.