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Summary. Histological studies ofadenohypophyses of male Friesian and Ayrshire calves, aged 1 to 2 months, and measurements ofplasma or serum LH by solid phase radioimmunoassay in Angus, Holstein, Guernsey and Jersey bulls, ranging in age from 7 months to 2 years were carried out to determine factors influencing gonadotrophin secretion. The gonado- trophin-secreting delta cells constituted 3 % of the cell population of the cortical region of the adenohypophyses at 1 month of age and 4 % at 2 months of age. Considerable quantities of LH (3-7 + 0-3 ng/ml) were present in plasma samples of Angus bulls at 7 to 9 months of age. The levels rose to 4-3+1-1 ng/ml at 12 to 14 months and, at 2 or more years of age, basal LH values were of the order of 7 to 10 ng/ml plasma or serum. No breed differences were noted either in pituitary histology or LH concentrations. Teasing and ejaculations caused no increase in serum LH values for up to 3 hr after sexual excitement, irrespective of whether a bull or an oestrous cow was the teaser. Large irregularly occurring daily fluctua¬ tions in serum LH (0 to 50 ng/ml) were found in 6-year-old Holstein bulls. Similar but smaller variations (1 to 16 ng/ml plasma) were found in the younger Angus bulls aged 7 to 14 months. A distinct 24-hr varia¬ tion in serum LH was found in two Holstein and two Guernsey bulls aged 1^ to 2 years. The diurnal serum LH values were twice the noc¬ turnal values, an abrupt change-over occurring at 18.00 hours when most of the bulls were lying down. Hourly variations were small and insignificant during the morning and afternoon.

REGULATION OF BLOOD LEVELS OF LH IN BULLS: INFLUENCE OF AGE, BREED, SEXUAL STIMULATION AND TEMPORAL FLUCTUATIONS