The relationship between ovulation and the changes in thyroid gland activity that occur during the oestrous cycle in rats, mice and hamsters.

1. When ovulation is prevented by the injection of pentobarbitone on the day of pro-oestrus in female rats, the expected increases in thyroid-serum concentration ratio (T/S ratio) for (131)I and in the uptake of (113)I by the thyroid on the day of oestrus do not occur.2. Ovulation induced by human chorionic gonadotrophin or ovine luteinizing hormone in rats treated with pentobarbitone at pro-oestrus is not associated with an increase in T/S ratio or (131)I uptake at oestrus.3. These results support the view that it is the neuro-endocrine changes at the hypothalamic or pituitary level that lead to ovulation rather than changes in the pattern of ovarian steroid secretion before or at ovulation that are responsible for the increased thyroid activity at oestrus in the female rat.4. Experimental evidence of an increase in thyroid activity after ovulation in mice and hamsters also supports this view.