The bromocriptine-TRH test was performed in 11 patients with advanced Parkinson's disease and in 10 control subjects. Seven patients were receiving treatment with Levo-DOPA, which was discontinued 12 h prior to the test. Basal prolactin levels were 5.88 ng/ml in treated and 18.22 ng/ml in untreated patients as compared to 9.68 mg/ml in controls. Bromocriptine induced a similar reduction of about 60% in prolactin levels in all groups (3.28, 11.5 and 5.95 ng/ml, respectively). After TRH, prolactin levels increased to 13.8 ng/ml in controls and 15.2 ng/ml in patients treated with levo-DOPA. Untreated patients remained with prolactin below basal levels (11.6 ng/ml). We postulate that low basal levels of prolactin in patients treated with levo-DOPA reveal a residual suppressing effect of the drug. The sharp increase after TRH is related to a significant dopaminergic hypofunction as evidenced by clinical findings. A high sensitivity of dopaminergic receptors in the anterior hypofisis is suggested as an explanation for the findings of the test in the untreated patients.