The present study was carried out to define whether dopaminergic mechanisms contribute to the regulation of thyrotropin (TSH) and prolactin PRL release in the immediate neonatal period. 14 full-term neonates with a mean birth weight of 3,240 g and a mean gestational age of 39.1 weeks were administered metoclopramide (MTC), a specific dopamine antagonist, in a dose of 0.1 mg/kg/day to treat delayed gastric emptying, regurgitation and abdominal distension. Prior to and after 3-day MTC administration, blood samples were taken to determine serum TSH and PRL levels using a radioimmunoassay method. It has been demonstrated that in response to MTC administration PRL increased significantly from 4,010 +/- 383 to 5,478 +/- 441 mU/l (p less than 0.01), while TSH showed only a tendency to rise independent of the pretreatment hormone levels (from 2.85 +/- 0.44 to 3.06 +/- 0.38 mU/L. In healthy control infants and in those infants with similar functional gastrointestinal disturbances who were treated without MTC, serum PRL levels fell significantly from days 3-4 to days 6-7, serum TSH, triiodothyronine and free thyroxine, however, remained unaltered. It is concluded that dopaminergic inhibitory mechanism may be involved in the control of pituitary PRL and TSH release already in the neonatal period.