Reproductive aging in female mammals is characterized by a progressive decline in fertility due to loss of follicles and reduced ovarian steroidogenesis. In this study we examined some of the endocrine and signaling parameters that might contribute to a decrease in ovulation and reproductive performance of mice with haploinsufficiency of the FSH receptor (FSH-R). For this purpose we compared ovarian changes and hormone levels in FSH-R heterozygous ( /−) and wild-type mice of different ages (3, 7, and 12 mo). Hormone-induced ovulations in immature and 3-mo-old /− mice were consistently lower. The number of corpora lutea (CL) were lower at 3 and 7 mo, and none were present in 1-yr-old /− females. The plasma steroid and gonadotropin levels exhibited changes associated with typical ovarian aging. Plasma FSH and LH levels were higher in 7-mo-old /− mice, but FSH levels continued to rise in both genotypes by 1 yr. Serum estradiol and progesterone were lower in /− mice at all ages, and testosterone was several-fold higher in 7-mo-old and 1-yr-old /− mice. Inhibin α (Western blot) appeared to be lower in /− ovaries at all ages. FSH-R (FSH* binding) declined steadily from 3 mo and reaching the lowest point at 1 yr. LH receptor (LH* binding) was high in the 1-yr-old ovary, and expression was localized in the stroma and interstitial cells. Our findings demonstrate that haploinsufficiency of the FSH-R gene could cause premature exhaustion of the gonadal reserves previously noted in these mice. This is accompanied by age-related changes in the hypothalamic-pituitary axis. As these features in our FSH-R /− mice resemble reproductive failure occurring in middle-age women, further studies in this model might provide useful insights into the mechanisms underlying ovarian aging.