Summary. The re-introduction of rams after a period of separation was used to stimulate LH secretion and induce ovulation in seasonally anovulatory ewes maintained under natural photoperiod. In 2 experiments, the rams were introduced in the morning or the evening to test for diurnal variations in responsiveness to the treatment. In the first experiment, with Romanov ewes, the ram-induced increase in tonic LH secretion was significantly earlier in the ewes treated (N = 6) at 07:30 h (mean ± s.e.m. delay to first pulse: 20 ± 6 min) than in those (N = 5) treated at 19:30 h (66 ± 15 min; P = 0·006). The pulse interval after the ram effect was significantly shorter in ewes that subsequently ovulated (120 ± 10 min) than in ewes that did not ovulate (288 ± 108 min; P = 0·043). There was a significant decline in pulse amplitude from 6·7 ± 1·2 to 3·4 ± 0·6 ng/ml (both groups combined) after the introduction of rams (P = 0·040). Of the 11 ewes, 7 subsequently ovulated and a preovulatory LH surge was observed in 6 of these 30–36 h after ram introduction.
In the second experiment, with seasonally anoestrous Préalpes-du-Sud ewes, the effect of the timing of the introduction of rams on the periovulatory events was tested. The delay to the onsets of oestrus and the LH surge was not affected, but the ovulation rate was higher after ram introduction in the morning (1·42) than in the evening (1·14).
In the 12-h period before the introduction of the rams in the first experiment, there was a difference between the groups in the secretion of LH, but the existence of diurnal rhythms in the concentrations of LH or FSH were not confirmed in a later study in which 7 ewes were sampled every 20 min for 36 h. In contrast, there was a distinct diurnal variation in the secretion of prolactin, with the highest values being recorded at night and the lowest around midday (P = 0·025). The rise and fall in prolactin values did not appear to coincide with dawn or dusk.
It is concluded that: (1) the hypothalamic centres controlling tonic LH secretion are more responsive to the introduction of rams in the morning than in the evening, an effect which appears to be associated with an increased ovulatory response; (2) that anoestrous Romanov ewes will respond to the ram effect with an increased frequency of LH pulses which leads to ovulation; (3) that the increased pulse frequency is accompanied by a decrease in pulse amplitude; (4) that the pulse frequency attained after ram introduction determines whether the ewe will ovulate.