Neurosteroids are known to modulate the development of the mesocorticolimbic system. We examined the effects of low (1 mg/kg) and high (10 mg/kg) doses of allopregnanolone on monoamine metabolism during the neonatal period (postnatal days 3–7) in rats. At 10 weeks of age, increases in homovanillic acid/dopamine (DA) ratios were found in the striatum in both allopregnanolone-treated groups compared with control rats. However, striatal DA levels decreased only in the low-allopregnanolone group and striatal homovanillic acid levels increased only in the high-allopregnanolone group. Allopregnanolone did not significantly affect cortical DA metabolism, or cortical or striatal serotonin metabolism. Data indicate that neonatal allopregnanolone treatment has a biphasic effect on striatal DA metabolism.