Effects of three dietary lysine (protein) concentrations during lactation on metabolic state, protein metabolism, reproductive hormones, and performance were investigated in 36 primiparous sows. Sows were assigned randomly to one of three diets containing .4% (low lysine, LL), 1.0% (medium lysine, ML), or 1.6% (high lysine, HL) total lysine from intact protein sources. All diets contained 2.1 Meal NE/kg and exceeded the recommended requirements for all other nutrients. Actual lysine intakes over an 18-d lactation were 16, 36, and 56 g/d for sows fed LL, ML, and HL, respectively. Fractional breakdown rate of muscle was determined on d 4 and 15 of lactation by using a three-compartment kinetic model of 3-methylhistidine metabolism. Increasing lysine intake during lactation did not affect fractional breakdown rate of muscle on d 4 of lactation but decreased it on d 15 (P < .05). Sows fed LL had a reduced number of LH pulses on d 12 and 18 (P < .05) and reduced serum estradiol (E2) concentration on d 18 of lactation compared with sows fed ML and HL treatments. However, LH pulses and E2 concentrations were similar between ML and HL treatments (P > .35). Increasing lysine intake increased serum urea nitrogen (SUN) and postprandial insulin concentrations (P < .05) during lactation but had no effect on plasma glucose concentrations (P > .20). Sows fed HL had greater serum IGF-I on d 6 and 18 than sows fed ML (P < .05). Number of LH peaks was correlated with serum insulin concen-tration 25 min after feeding on d 6 and 18 (r = .31 to .41; P < .1) and pre- (r = .33 to .46) and postprandial (r = .30 to .58) SUN concentrations (P < .05) during different stages of lactation. Results indicate that, compared with medium lysine intake, low lysine intake increased muscle protein degradation and decreased concentrations of insulin, SUN, and estradiol and LH pulsatility. In contrast, high lysine (protein) intake increased SUN, insulin, and IGF-I, but did not increase secretion of estradiol and LH compared with medium lysine intake. Furthermore, nutritional impacts on reproduction may be mediated in part through associated effects on circulating insulin concentration.