Hypothalamic dopamine-β-hydroxylase (DBH) activity and plasma luteinizing hormone (LH) concentrations were analyzed in female C57BL/6 mice aged 2–4, 7–8, 12–13, and 16–20 months at various times of the estrous cycle. Hypothalamic DBH levels were low during diestrus and metestrus in all age groups. In animals aged 2-4, 7-8, and 12–13 months DBH activities started to rise on the evening of proestrus, and the highest levels attained at 21:00 h were significantly greater than those found during the morning (11:00 h) of proestrus. In contrast, in 16- to 20-month-old mice the hypothalamic DBH activity did not show any significant proestrous rise. In this age group the highest levels of DBH were present at 24:00 h, but these were not significantly different from those found at 11:00, 18:00 or 21:00 h of proestrus. Plasma LH concentrations in all age groups began to increase at 18:00 h of proestrus and had reached their highest levels by 21:00 h of proestrus. The plasma LH concentrations at 21:00 h of proestrus were significantly higher when compared with those 3 h earlier in mice aged 2–4, 7–8, and 12–13 months; however, the LH values in 16- to 20-month-old mice were not significantly different when comparing these two time periods. These data are consistent with (1) the existing idea of an increased adrenergic activity in the rodent hypothalamus during the preovulatory proestrous period and (2) the hypothesis that a dysfunctioning female reproductive system in older rodents may result from a reduction in hypothalamic catecholaminergic activity. It is suggested from the experimental results that such a reduction in adrenergic activity in the mouse hypothalamus may be, at least in part, caused by a reduction in the levels of the norepinephrine-synthesizing enzyme, DBH.