Based on the reported health benefits of flaxseed, many Canadians are choosing to consume flaxseed or flaxseed-containing foods. However, the safety of exposure to flaxseed during early life such as the suckling period has not been studied, despite the fact that components in flaxseed with potential hormone-like effects can be transferred to nursing offspring via mother's milk. Previous investigations demonstrated that maternal feeding of a 10% flaxseed diet during pregnancy and lactation resulted in estrogenic effects on reproductive indices among male and female offspring. These effects were attributed to the potential estrogenic activity of enterodiol and enterolactone, the two major mammalian lignans that are converted from secoisolariciresinol diglycoside (SDG) in flaxseed by colonic bacteria; however, the effect of exposure to purified SDG at the level of a 10% flaxseed diet was not studied. The objective of this study was to determine whether maternal feeding of flaxseed during lactation altered reproductive indices in male and female offspring. Rat dams were fed basal diet (BD) or BD containing either 100% flaxseed (10F) or the equivalent quantity of SDG present in the 10% (10S) flaxseed diet from the start of lactation until pups were 21 d old. At the end of lactation (postnatal day IPND] 21), suckling pups either continued on the mother's diet or were switched to BD until adolescence (PND 50) or young adulthood (PND 132) to determine if continuous exposure to flaxseed or SDG altered reproductive indices. The reproductive indices that were measured included anogenital distance from birth through PND 21, age and body weight at puberty onset (females only), estrous cycle length, reproductive organ weights at PND 50 and 132, and histological analysis of reproductive organs (uterus, ovaries, prostate) at PND 132. There were no significant effects of exposing male or female offspring to flaxseed or SDG during suckling only or during suckling through the postsuckling period on any of the reproductive indices measured. These findings are in contrast to the estrogenic effects observed in male and female offspring exposed to flaxseed during fetal life through suckling and suggest that fetal life is a more hormone-sensitive period of development. Although maternal feeding of flaxseed during lactation appears to be safe with respect to reproductive indices among offspring, future investigation is required to elucidate whether there are any long-term implications with respect to fertility.