A few years before reproductive senescence, primordial follicles are depleted from the ovary at a dramatically accelerated rate. It has been proposed that this depletion is due to transient increases in gonadotropin levels. To test this hypothesis, we used mice that produce chronically elevated levels of serum LH via expression of an LHbeta subunit transgene. Ovaries were collected from transgenic and control mice, and complete serial sections were prepared for histological examination. Each section was scanned for morphological abnormalities, and every fifth section was sampled to estimate the total number of primordial, primary, and large preantral follicles per ovary. Until 3 wk postpartum, ovaries from transgenic and control mice were morphologically similar. By 5 wk, control ovaries contained many healthy primordial, primary, and large preantral follicles as well as atretic follicles. Transgenic ovaries contained blood-filled cysts, misshapen granulosa cells, luteinized cells, and approximately 45% fewer primordial follicles than controls. By 3 mo, transgenic ovaries had about 68% fewer primordial follicles and 53% fewer primary follicles than controls. These results suggest that, in addition to having profound effects on growing follicles, chronically elevated LH levels deplete the primordial follicle pool and thus may hasten the onset of reproductive senescence.