To explore the effect of metoclopramide (MC) on the secretion of PRL, TSH, and thyroid hormones (T3 and T4) and on defective lactation, 17 mothers with poor lactation were treated with oral MC (10 mg. three times daily) for 3 weeks starting 18-141 days post partum. After a pause of 1 week, the medication was given for a further 2 weeks. The breast milk yield was monitored objectively before and during the trial. Furthermore, iv stimulation tests with MC (10 mg) and TRH (200 microgram) were done before and at the end of oral MC therapies. Oral MC increased the mean (+/-SEM) plasma PRL level from 36.6 +/- 9.2 to 90.6 +/- 7.5 ng/ml (P less than 0.001) after 1 week, and the PRL level remained elevated for as long as MC was administered. During the pause, the PRL level decreased to 19.5 +/- 7.5 ng/ml, but increased once again during the second MC treatment to 85.5 +/- 16.0 ng/ml (P less than 0.01). Plasma TSH, T3, and T4 did not change. The PRL level rose significantly after TRH and MC injections before and during oral treatments with MC, whereas the TSH concentrations were elevated only after TRH stimulation. The PRL response to iv MC or TRH and the TSH response to iv TRH were not affected by oral MC treatment. The mean daily milk volume increased from 433 +/- 55 to 626 +/- 75 ml (P less than 0.001) during the first treatment and from 390 +/- 73 to 606 +/- 56 ml (P less than 0.01) during the second oral MC treatment. Correspondingly, the volume of daily supplemental alimentation decreased from 348 +/- 61 to 280 +/- 59 ml (P less than 0.05) and from 526 +/- 68 to 363 +/- 66 ml (P less than 0.01), respectively. MC caused no significant side effects.