Summary. The functioning of the stimulatory oestrogen feedback mechanism during lactational anoestrus of the sow was evaluated by determining the LH response to a single i.m. injection of oestradiol benzoate. Oestradiol-17β and progesterone levels were also measured following treatment and following weaning. On Day 5 of lactation there was no LH response to oestradiol benzoate but on Day 35 of lactation a small but significant (P ≤ 0·05) increase in plasma LH occurred, and this was followed by a rise in plasma progesterone in 4 out of 9 animals 8 or 13 days later. It is concluded that blockade of the stimulatory oestrogen feedback mechanism is one cause of lactational anoestrus in the sow.