**4.2 Reproductive aging and the ovary**

In the present study we showed that proestrous P levels were comparable between 4- and 8.5-month-old rats, and thus do not underlie the observed attenuation of the LH surge. In contrast, another study (Miller and Riegle, 1980) showed that the attenuated preovulatory LH surge was accompanied by an attenuated P surge in 12-month-old cyclic Long-Evans rats. It has been suggested that attenuated P levels result from a decrease in proestrous LH levels, although increased responsiveness of the ovary to hCG stimulation in regular cyclic middle-aged compared has been reported for Long-Evans rats (Chern et al, 2000). Consequently, the lack of concurrent changes in P and LH release in our rats could be explained by an increased responsiveness to LH stimulation.
