Young adult (3 months old) and old (26\p=n-\28 months old) male Wistar rats were studied. Testicular weight was 1.66 g (range: 1.33 \p=n-\ 2.86) in the younger group and 1.62 g (range: 1.04 \p=n-\ 1.90) in the older group. The total Leydig cell volume as measured by a quantitative histometric method was significantly larger in the old animals (\l =x_\ = 0.153 ml vs. \l=x_\ = 0.089 ml). The testicular HCG binding capacity was 2.54 ng HCG per 100 mg tissue (range: 1.88 \p=n-\3.77) in the younger animals and 1.83 ng HCG per 100 mg tissue (range: 0.80 \ p=n-\ 3.02) in the older ones (P < 0.01). Plasma testosterone was on an average 242 ng/100 ml (range: 72 \ p=n-\1162) in the young adult rats and 91 ng/100 ml (range: 23 \p=n-\277) in the older rats. Plasma LH was only slightly (P < 0.05) decreased and was 49 ng LH\p=n-\RP\p=n-\1/ml(range: 14 \p=n-\ 120) in the younger group and 40 ng LH\p=n-\ RP\p=n-\1ng/ml (range: 5 \ p=n-\ 98) in the older.

The hypothalamic-pituitary gonadal axis was studied in young adult (3 month old) and old (24 to 27 month old) male Wistar rats. Plasma testosterone decreased significantly in old animals (\l =x_\: 262 ng/100 ml (n = 35); versus \l=x_\:110 ng/100 ml (n = 30)). The fall in LH was less pronounced but still significant (54.5 ng LH-RP-1/ml in young versus 39.5 ng/ml in old rats). Groups of 6 to 8 animals of both ages were castrated and implanted with silastic capsules continuously releasing testosterone. The length of the capsules was directly proportional to the plasma testosterone levels achieved (range between 63 and 350 ng/100 ml).After one week young castrated rats not substituted with testosterone showed LH values three times higher (\l =x_\: 351 ng/ml) than old rats treated in the same way (\l =x_\ = 126 ng/ml). LH values in the animals substituted with testosterone indicate that the sensitivity of the negative testosterone\x=req-\ LH feedback is greatly increased in old rats. Testosterone can be depressed to 60 ng/100 ml before an increase in LH occurs. In young rats no increase in LH was observed when testosterone values were higher than 170 ng/100 ml. In the range between 170 and 100 ng/100 ml about half of the young animals reacted with increased LH secretion, while an increase was observed in all young animals when testosterone dropped below 100 ng/100 ml.

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