Diurnal variation in release of LH and testosterone in the ram

Summary. The time of appearance of plasma LH and testosterone peaks through the day was determined in 75 Pr\l=e'\alpesdu Sud and 41 Ile-de-France rams in December and in 44 Pr\l=e'\alpesdu Sud and 11 Ile-de-France rams in June. The distribution of peaks throughout the day was non-random for the two hormones in the two breeds and for both times of the year (P < 0\m=.\01at least on each occasion; P< 0\m=.\001on pooled data from the two breeds). The most striking features were the occurrence of(1) a minimum of LH and testosterone peaks immediately after 'dawn ' (lights on) in both months; (2) a maximum of peaks 3 h after 'dawn ' in June and 4 h after 'dawn ' in December. For several hours after the increase in frequency of peaks the probability of measuring peaks of LH and testosterone remains high. This and the correlation between the LH values in December and June when adjusted for the time of 'dawn' suggest that dawn could act as a synchronizer of gonadotroph activity