The relationship between the return of menstruation and ovulation and the pattern of breast-feeding was studied in a group of 29 women in a rural area of Mexico; eight non-breast-feeding post-partum women were also studied. The return of ovulation was assessed by weekly determinations of urinary pregnanediol. The number of suckling episodes and supplementary feeds was recorded. The cumulative probability of resumption of menses for the breast-feeders was 28% and 52% at 3 and 6 months, respectively, and for the non-breast-feeders was 75% and 88%. The cumulative probability of ovulation for the breast-feeders was 28%, 66%, 69% and 72% at 3, 6, 9 and 12 months, respectively, and for the non-breast-feeders was 50% and 100% at 3 and 6 months. The earlier the introduction of supplements the earlier the return of ovulation. The nonovulatory subjects maintained a higher frequency of suckling episodes than did the ovulatory subjects. The cumulative probability of pregnancy for the breast-feeders was 3%, 7%, 7% and 10% at 3, 6, 9 and 12 months, respectively, and for the non-breast-feeders was 13%, 38% and 50% at 3, 6 and 9 months.