Serial plasma oxytocin (OT), PRL, TSH, FSH, LH, estrone, estradiol, and progesterone were measured by RIA in 12 women before and during a 30-min breast-feeding period on the third or fifth postpartum day. Plasma OT increased significantly from 10.8 +/- 3.4 to 22.4 +/- 3.5 pg/ml (mean +/- SE) within 2 min of suckling (P = less than 0.05) to reach the mean peak level of 53.2 pg/ml at 10 min. The increase in plasma OT was bimodal. Plasma PRL and TSH also increased significantly from baseline levels of 192 +/- 39 ng/ml and 16.9 +/- 5.6 microU/ml, respectively, to reach maximum levels of 427 +/- 91 ng PRL/ml at 10 min (P = less than 0.025) and 281.5 +/- 56.6 microU TSH/ml at 25 min (P = less than 0.005). Plasma FSH-beta (range of means, 3.5-4.6 ng/ml), LH (range of means, 1.7-2.6 mIU/ml), and estradiol (range of means, 29.8-38.2 pg/ml) were low and remained unchanged throughout breast feeding. Plasma progesterone was 6.0 +/- 0.4 ng/ml before breast feeding and did not alter significantly during breast feeding. The significance of these findings is discussed in relation to the milk let-down reflex and the relationship of TSH to PRL.