The purpose of the present study was to characterize pharmacologically dopamine D1 receptor-mediated inhibition of tuberoinfundibular dopamine neurons in males rats, and to determine if inhibitory dopamine D1 receptors oppose stimulatory dopamine D2 receptors and account for the inability of mixed dopamine receptor agonists to alter the activity of these neurons. Tuberoinfundibular dopamine neuronal activity was estimated by measuring the concentrations of the dopamine metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the median eminence, the region of the hypothalamus containing terminals of these neurons. Administration of the dopamine D1 receptor agonist (+/-)-1 phenyl-2,3,4,5-tetrahydro-(1 H)-3-benzazepine-7,8-diol (SKF38393) decreased median eminence DOPAC and increased plasma prolactin concentrations, whereas administration of the dopamine D1 receptor antagonist ((-)-trans,6,7,7a,8,9,13b-hexahydro-3-chloro-2-hydroxy-N-methyl-5H -benzo[d]naphtho-[2,1 b]azepine (SCH39166) increased median eminence DOPAC concentrations but had not effect on plasma prolactin. The inhibitory effect of SKF38393 on median eminence DOPAC concentrations was blocked by SCH39166. These results demonstrate that acute activation of dopamine D1 receptors inhibits the activity of tuberoinfundibular dopamine neurons and thereby increases prolactin secretion, and that under basal conditions dopamine D1 receptor-mediated inhibition of tuberoinfundibular dopamine neurons is tonically active. Administration of the dopamine D2 receptor agonist (5aR-trans)-5,5a,6,7,8,9,9a,10-octahydro-6-propyl-pyridol[2, 3-g]quinazolin-2-amine (quinelorane) increased median eminence DOPAC concentrations, and SKF38393 caused a dose-dependent reversal of this effect. Administration of the mixed dopamine D1/D2 receptor agonist R(-)-10,11-dihydroxy-apomorphine (apomorphine) had no effect per se, but blocked quinelorane-induced increases in DOPAC concentrations in the median eminence. These results reveal that concurrent activation of dopamine D1 and D2 receptors nullifies the actions of each of these receptors on tuberoinfundibular dopamine neurons, which likely accounts for the lack of an acute effect of mixed dopamine D1/D2 receptor agonists on these hypothalamic dopamine neurons.