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In the 5-day cyclic rat the ovarian androgen and oestrogen concentrations, measured by radioimmunoassay, started a gradual rise on the day of metoestrus, reaching maximal values in the period from 22.00 h on dioestrus 2 until 17.00 h on the day of pro-oestrus. A steep decline in the ovarian androgen and oestrogen concentrations occurred between 17.00 and 21.00 h on the day of pro-oestrus.

Rats, injected intraperitoneally with an ovulation-blocking dose of pentobarbitone sodium (35 mg/kg body weight) at 13.00 h on the day of pro-oestrus, tended to show a decline in ovarian concentrations of androgens and oestrogens during late pro-oestrus, but this decrease was less steep than in saline-injected rats. At 10.00 and 14.00 h on the day after pro-oestrus the ovarian concentrations of androgens and oestrogens in rats treated with pentobarbitone sodium showed a further decline but values were still higher than those in saline-treated oestrous rats. At 17.00 h, i.e. 28 h after injection with pentobarbitone sodium, a distinct increase in the ovarian androgen and oestrogen concentrations was noted and a steep decline was observed at 21.00 h on the day after pro-oestrus.

Thus normal as well as delayed ovulation was preceded first by an increase and then by a steep decline in ovarian concentrations of oestrogens and androgens.