Plasma luteinizing hormone (LH), progesterone, testosterone and estradiol concentrations were measured from the same 10 male and 10 female chickens at various ages of 28 to 1141 days. They were kept on a photoperiod of 14L: 10D (lights on 0500-1900h) and bled at 1400h.
In the female, plasma levels of LH and estradiol rose from 4 weeks of age to sexual maturity, and both reached peaks shortly before the onset of egg production, and dropped radidly thereafter. Either plasma progesterone or testosterone showed a similar change to that of LH or estradiol, except that the two hormones reached their peaks shortly after the onset of egg producttion. The plasma LH level dropped a little but was maintained at a fairy high level after one year of age, whereas the progesterone level dropped and reached a trough at one year of age, and fluctuated thereafter. The plasma testosterone level was very low and showed a sudden fall at one year of age, and remained at a low level thereafter. The plasma estradiol level showed a rise after one year of age, and fluctuated thereafter. The egg production rate reached a peak a month after the onset of laying and decreased with the advance of age. Positive correlations were observed between the egg production rate and LH, progesterone and testosterone concentrations, and a negative one between the egg production rate and estradiol concentration. It is postulated that the decrease in the egg production rate due to age is caused by a decrease in ovarian function rather than decrease in pituitary gonadotropic function.
In the male, LH levels rose during the prepuberty stage, reached the first peak at 238 days of age, and fluctuated thereafter with 3-5 fold higher values than those in the female at corresponding ages. Plasma progesterone levels fluctuated and showed four peaks, the values being higher than those of the female. Plasma testosterone levels were 10-20 times as high as those in the female, being low before sexual maturity, and reaching the first peak 2-3 months after sexual maturity. Plasma estradiol in the male was not detectable throughout the entire experimental period.