The timing and dosage of estradiol benzoate injected after weaning was critical with respect to the pattern of behavioral estrus and the ovarian stimulation achieved; treatment on the day of weaning (day 0) and day 1 with 60 .mu.g estradiol benzoate/kg body wt produced poor ovulatory responses and abormal estrous behavior. Treatment on day 2 with 30 .mu.g estradiol benzoate/kg resulted in consistent estrus and ovulatory responses although the ovulation rates (10.6 .+-. 1.1 in 3-wk and 12.2 .+-. 1.7 in 5-wk weaned sows) were below those expected in fertile untreated sows weaned at these times. The timing of the preovulatory LH [luteinizing hormone] surge (53.6 .+-. 2 h after estradiol benzoate) was closely synchronized in all sows and a similar synchronous rise in plasma progesterone concentrations 100 .+-. 4 h after estradiol benzoate suggests a similar synchrony of ovulation. The magnitude of the LH and FSH responses to estradiol benzoate were similar to those that occur in untreated sows and similar differences also existed in gonadotropin secretion related to the length of lactation.