During a 21-day lactation period, 48 primiparous sows were fed a fat- or a starch-rich diet (131 g fat and 183 g starch + sugar x kg(-1) vs. 31 g fat and 351 g starch + sugar x kg(-1)) at a high or a low level of feeding (44 vs. 33 MJ net energy per day) according to a factorial design. Blood samples were collected at days 7, 14, 21, 22 (weaning), 24, 25, 26, and 27 post-partum (p.p.). IGF-I levels were higher with the starch-rich than with the fat-rich diet at days 7, 21, 22, and 24 p.p. Plasma IGF-I concentrations on day 22 p.p. were positively related with LH pulse frequency on day 22 p.p. and the height of the pre-ovulatory LH surge. Sows with low body weight at farrowing and high lactational body weight loss had lower plasma IGF-I concentrations than others, before and after weaning. These results indicate that IGF-I concentrations are affected by both feeding level and dietary energy source and are related to the secretion of LH. Furthermore, body weight at farrowing interacts with lactational body weight loss to affect IGF-I concentrations.