Objective(s):To investigate the pattern of follicular development in transplanted fetal and neonatal mouse ovaries in the presence or absence of the recipient's own ovaries.Design:Controlled experiment.Setting:Academic research laboratory, Department of Physiology, Monash University, Clayton, Australia.Intervention(s):Mouse ovaries from 16-day-old fetuses, 3-day-old neonates, and 10-day-old neonates were transplanted under the kidney capsule of adult female mice, which either retained their own ovaries in situ or were bilaterally ovariectomized.Main Outcome Measure(s):Histologic analysis.Result(s):By 4 weeks after transplantation, fetal and neonatal ovaries transplanted to ovariectomized recipients displayed a pattern of follicular development similar to that observed in in situ adult mouse ovaries. In contrast, follicular development did not progress beyond the early antral stage in fetal and 3-day-old ovaries transplanted to recipients that retained their in situ ovaries. However, 10-day-old ovaries transplanted to recipients that retained their in situ ovaries displayed full follicular development and corpora lutea formation by 8 weeks after transplantation.Conclusion(s):Follicular development in transplanted immature ovarian tissue is influenced by the age of the donor ovary and gonadal status of the recipient. (Fertil Steril(R) 2000;74:000-00. (C)2000 by American Society for Reproductive Medicine.)