Normal ranges of urinary pregnanediol and total urinary oestrogen were determined from weekly estimations in twenty-seven cycles from seven normally menstruating control women and compared with the levels in the cycles of twenty-seven breast feeding and ten bottle feeding mothers. During lactation, the luteal phase pregnanediol levels were within normal limits in thirteen of forty-nine (27%) cycles, the remainder of the luteal phases being deficient (31%) or absent (42%). The proportion of normal luteal phases remained low during first cycles after lactation in six of twenty-three (26%) but rose to twenty-four of thirty-one (77%) in subsequent cycles. In bottle feeding mothers, the luteal phases of first post-partum cycles were deficient in two often (20%) and absent in eight of ten (80%). In second post-partum cycles, the majority of luteal phases were deficient (eight often, 80%) and it was not until third post-partum cycles that seven of eight (88%) had luteal phase pregnanediol levels in the normal range. Cycles during lactation and first cycles after lactation had significantly lower mean urinary pregnanediol and total urinary oestrogen levels than both the control cycles and the later cycles after lactation. Similarly, first post-partum cycles in bottle feeders had lower urinary pregnanediol and total urinary oestrogen levels than controls. This study shows an increased frequency of abnormal luteal phases during the early post-partum menstrual cycles of both breast and bottle feeding mothers which may be associated with defective development of the follicle.