Mitochondria and sex steroid hormones during aging. Mitochondria produce sex steroid hormones in the gonads through initial conversion of cholesterol to pregnenolone. Sex steroid hormones improve and/or maintain mitochondrial function in hormone responsive cells by regulating: 1. gene expression of nuclear-encoded mitochondrial proteins, 2. gene expression of mitochondrial-encoded mitochondrial proteins, and/or 3. activity of mitochondrial proteins. Increased oxidative stress during aging damages gonadal cells and/or impair steroidogenesis. Decline in sex steroid hormone biosynthesis during aging compromises mitochondrial function in hormone responsive tissues and contribute to age-related pathologies.