The pharmacokinetics and endocrinological effects of metoclopramide were investigated in 5 mothers with deficient lactation and in their children soon after delivery. In addition, the transfer of metoclopramide into breast milk was evaluated in 18 mothers during the 8th to 12th puerperal weeks. Metoclopramide was detected in all the milk samples studied, generally at a higher concentration than in maternal plasma. Metoclopramide was found in plasma from only 1 of the 5 neonates studied. Exposure of the child to metoclopramide, estimated by multiplying the daily breast milk volume by the concentration of metoclopramide in the milk, ranged from 6 to 24 micrograms/kg/day for the 5 children in the early puerperium to 1 to 13 micrograms/kg/day for the 18 children during the late puerperium. These quantities are considerably less than the therapeutic dose of 500 micrograms/kg/day recommended for children. However, the plasma concentration of prolactin in 4 out of 7 neonates sampled taken during administration of metoclopramide to the mother were higher than the highest plasma prolactin level in children of same age of untreated mothers. The plasma concentration of thyrotrophin in the newborns remained within the normal range.