A double immunofluorescence technique has been used to assess postnatal maturational changes in the extent to which dopamine-immunoreactive (DA-IR) varicose fibers form contacts with gamma-aminobutyric acid (GABA)-immunoreactive (IR) neuronal cell bodies in rat medial prefrontal cortex (mPFCx). Two separate measures of interaction, the percentage of GABA-IR cell bodies having DA-IR varicosities in apposition and the number of such profiles in contact with any given GABA-IR cell, were assessed. Between birth and adulthood, there was a progressive linear increase (r = 0.75, P < or = 0.0005) in the percentage of GABA-IR cell bodies having at least one DA-IR varicosity in apposition. While the number of varicosities in contact with any given GABA cell body showed very little change during the preweanling period, later during the postweanling interval, this parameter increased in a curvilinear fashion toward adult levels (r = 0.81, P < or = 0.0005). Taking together these latter two measures, an index of interaction was found to be 1.8 times higher when adult animals were compared to postweanling rats, and 2.5 times higher when compared to preweanling rats. Overall, these results are consistent with the view that there are late postnatal changes in the extent to which midbrain DA afferents interact specifically with GABAergic interneurons in rat mPFCx.