Summary. Primiparous sows were fed to appetite during lactations that occurred during winter or summer, and 11·4 ± 0·4 pigs per litter were weaned at 23·5 ± 0·1 days of age. Sows were slaughtered at 0 or 72 h after weaning or blood samples were collected until 24 h after onset of oestrus. Sows that lactated during summer consumed less food and lost more (P < 0·05) weight, heartgirth and backfat than those that lactated during winter. Weaning-to-oestrus interval was greater (P < 0·05) in summer (224 ± 25 h) than in winter (93 ± 13 h). Content of GnRH in the hypothalamus and concentrations of LH in the anterior pituitary and serum were lower (P < 0·05) after weaning in summer than winter. The numbers of visible ovarian follicles < 5 mm in diameter at weaning were lower (P < 0·05) in summer than in winter. In contrast to LH, FSH concentration in serum was higher (P < 0·10) in summer than winter, but FSH values in the anterior pituitary were lower (P < 0·05) in summer than in winter. Post-weaning patterns of secretion of oestradiol and follicular development differed between winter and summer. For example, in some sows weaned during the summer, transient surges of oestradiol occurred repeatedly during 0 to 280 h after weaning without provoking surges of LH. These results indicate that the period of post-weaning anoestrus in summer is prolonged because of altered activity of the hypothalamic– pituitary axis, possibly because of changes in sensitivity to the feedback of oestradiol. Lower feed intake during lactations that occur during summer may predispose the endocrine system to the aberrations.