Lactating, primiparous Landrace x Yorkshire sows were used to characterize LH secretion during lactation in sows that experienced an early (less than 9 d; n = 14) or late (greater than 15 d; n = 9) return to estrous postweaning and to evaluate the relationship between LH secretion and blood metabolites. Twenty-three sows were fed one of nine corn-soybean meal diets to achieve a matrix of lysine (15 to 45 g/d) and energy (6.5 to 16.5 Mcal of ME/d) intakes and a range in metabolite concentrations and return-to-estrus intervals. Blood samples for LH analysis were collected every 15 min for 6 h on d 0, 7, 14, 21, and 28 of lactation. Circulating concentrations of glucose, amino acids, insulin, triglycerides, urea N, and nonesterified fatty acids also were measured on d 7 and 21. Mean LH concentrations were .27 and .42 ng/mL at farrowing for sows with an early and late return to estrus, respectively, but decreased (P less than .01) to .12 ng/mL by d 7 in both early and late groups. Mean LH and number of LH peaks per 6 h increased linearly (P less than .01) from d 7 to 28 for early sows. Early sows had a higher LH mean and more LH peaks per 6 h on d 14, 21, and 28 than did late sows (P less than .05). Early sows had higher serum insulin on d 7 (P less than .05) and d 21 (P less than .01) than did late sows. Concentrations of other metabolites did not differ (P less than .10) between early and late sows.(ABSTRACT TRUNCATED AT 250 WORDS)