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1. A single subcutaneous injection of 5 mg, 1·25 mg or 625 µg, but not 312 µg, of progesterone in oil delayed ovulation in the rat by 1 or more days, when injected at the dioestrous stage of a 4-day oestrous cycle.

2. When ovulation was delayed in this way the expected increase in the thyroid-serum concentration ratio for ¹³¹I was also delayed but the ratio did increase when delayed ovulation occurred.

3. A single injection of progesterone resulted in an increase in the uterus-plasma and oviduct-plasma concentration ratios for ¹³¹I; the increase was greatest when steroid was injected at the dioestrous stage of the cycle and was delayed and least when the steroid was given at the pro-oestrous stage.

4. Ovulation was advanced by 1 day when progesterone was injected on the second day of dioestrus in rats showing regular 5-day cycles; this ovulation was not accompanied by an increase in the thyroid-serum concentration ratio. In these experiments a dose of progesterone that failed to advance ovulation produced a rise in uterus-plasma and oviduct-plasma ratio for ¹³¹I but no rise was seen when ovulation was induced, suggesting that oestrogen secretion had been stimulated.

5. 20-Dihydroprogesterone (pregn-4-en-20-o1-3-one) was not effective in delaying or advancing ovulation at a dose level of 2·5 mg per rat and had no effect on the uterus-plasma concentration ratio for radio-iodide.

6. These results are discussed in relation to the hypothesis that the increase in thyroid gland activity at the oestrous stage of the cycle is related to the neuro-endocrine changes that lead to ovulation.