To the Editor: Musey et al. (Jan. 29 issue)1 speculated that the long-term decrease in serum prolactin after pregnancy may be related to the protective effect against the development of breast cancer associated with an early first pregnancy. They reported significantly lower serum prolactin levels in healthy premenopausal parous women as compared with nulliparous women studied during the early follicular phase of the menstrual cycle. These data strongly support our observation that parous women have significantly lower plasma prolactin levels than nulliparous women, as measured in blood obtained by continuous venous withdrawal from 3 to 11 p.m. during the luteal. © 1987, Massachusetts Medical Society. All rights reserved.