The actions of parenteral morphine were examined with regard to dopamine metabolism in the mesocortical dopaminergic pathways of the rat. The effects of morphine on dopamine metabolism in the prefrontal, cingulate, pyriform and entorhinal cortices were compared with the actions of morphine on the metabolism of dopamine in the striatum and olfactory tubercle. In all tissues, except the entorhinal cortex, morphine significantly elevated the dopamine metabolites dihydroxphenylacetic acid and homovanillic acid. These data, along with previous studies of various pharmacological agents, clearly indicate that the mesocortical dopaminergic projections possess unique opioid and non-opioid regulatory inputs.