Plasma PRL was measured at 20-min intervals in six patients with Parkinson's disease under various treatment protocols. In addition, 24-h mean GH levels were measured. The results of these studies showed that two untreated patients with Parkinson's disease had normal 24-h mean PRL levels with the normal increase during sleep. During chronic treatment with L-dopa-carbidopa (Sinemet), the 24-h PRL level was 12.8 +/- 4.9 ng/ml (mean +/- SD) and there was persistence of augmented PRL secretion during sleep. The 24-h mean GH level ranged from 1.5-4.4 ng/ml, with a mean of 2.5 ng/ml. The addition of a dopamine agonist (Lergotrile mesylate) resulted in a significant (P less than 0.01) suppression of the 24-h mean PRL levels and abolition of the normal sleep augmentation after 2 weeks of therapy. This suppression was maintained in one patient who was restudied 4 months after the addition of dopamine agonist therapy to L-dopa-carbidopa. The 24-h mean GH levels did not change significantly after the addition of the dopamine agonist when compared to L-dopa-carbidopa alone. These results suggest a dichotomy between the PRL and GH responses to combined L-dopa-carbidopa and dopamine agonist therapy. In addition, the preservation of normal PRL regulation in the two untreated patients with Parkinson's disease suggests that dopaminergic neurons are not universally affected in this disorder.