Neurochemical substrates of intravenous opiate self-administration were investigated in rats using littermate controls for vehicle and passive morphine infusion. The rates of turnover of the putative neurotransmitters, dopamine, norepinephrine, serotonin, gamma-aminobutyric acid, aspartate and glutamate were concurrently measured in eleven brain regions of rats intravenously self-administering morphine and yoked-morphine or yoked-vehicle infused littermates. The passive infusion of morphine resulted in significant changes in the rates of turnover of the biogenic monoamine and amino acid neurotransmitters in six brain regions with the caudate nucleus-putamen-globus pallidus showing the most changes. The contingent infusion of morphine resulted in changes in utilization rates that were generally greater in both magnitude and number than the effects of the drug itself. Twenty-nine significant changes were observed in the self-administering group with most changes occurring in limbic structures. The neurotransmitter turnover rate changes resulting from contingent administration suggest that the drug administration environment is an important factor that should be considered in studies of interactions between drugs and neuronal systems.