Pituitary secretion of PRL and TSH is under the control of inhibitory dopaminergic and stimulatory TRH-mediated mechanisms. To evaluate the relationships between these regulatory systems, ten healthy women were treated with oral TRH (20 mg twice daily), a dopamine blocking drug, metoclopramide (MC) (10 mg t.d.s.) or placebo for 1 week (from 8th to 14th cycle day). Serum concentrations of PRL, TSH, T3 and T4 were determined before, at the end, and 3 days after the treatments. In addition, PRL and TSH responses to i.v. TRH (200 micrograms) or MC (10 mg) were studied at the end of the oral treatments. Oral TRH treatment was accompanied by increases in basal T3 and T4 concentrations, no change in PRL, and a decrease in TSH 3 days after the end of treatment. Oral TRH did not modify the PRL response to i.v. MC while it eliminated the TSH response to i.v. MC, possibly because of elevated concentrations of thyroid hormones. Oral MC treatment raised the concentrations of PRL, T3 and T4, and also potentiated the PRL response to i.v. TRH, whereas the TSH response remained unaltered. These results demonstrate that dopaminergic and TRH-mediated mechanisms are related in the control of PRL and TSH secretions, perhaps directly or through thyroid hormones.