The MtTW15 transplantable pituitary tumour grown in rats was tested in vitro for the ability of dopamine agonists to affect prolactin secretion and for the existence of dopamine receptors. Prolactin release from enzymatically dispersed cells and non-enzymatically treated tissue fragments of both the tumour and the anterior pituitary gland was determined in a cell perifusion column apparatus. Dopamine (0.1-5 μmol/l), bromocriptine (50 nmol/l) and the dopamine antagonist haloperidol (100 nmol/l) had no effect on prolactin release from the tumour cells. In contrast, dopamine (500 nmol/l) inhibited prolactin secretion from normal anterior pituitary cells in a parallel cell column and haloperidol blocked this inhibition. Although oestrogen treatment in vivo stimulated prolactin release in vitro when the tumour was removed and studied in the cell column, oestrogen had no effect on the inability of dopamine to modify the prolactin secretion. Growth hormone release from the tumour cells was not affected by dopamine. Although MtTW15 cells were refractory to dopaminergic inhibition of prolactin release, the dopamine receptors present in tumour homogenates were indistinguishable from the dopamine receptors previously defined in the normal anterior pituitary gland. The binding of the dopamine antagonist [3H]spiperone to the tumour was saturable (110 fmol/mg protein), of high affinity to one apparent class of sites (dissociation constant = 0.12 nmol/l), reversible and sensitive to guanine nucleotides. The pharmacology of the binding was defined in competition studies with a large number of agonists and antagonists. From the order of potency of these agents, a dopaminergic interaction was apparent. We conclude that the prolactin-secreting MtTW15 tumour cells appear to to be completely unresponsive to dopamine or to the potent agonist bromocriptine, in spite of apparently normal dopamine receptors in the tumour.