Tryptophan (Trp) as a precursor of serotonin (5-hydroxytryptamine, 5-HT) has long been used to extenuate aggressive behavior and control stress of humans as well as several farm animals. This study was conducted to determine the effect of supplemental L-Tryptophan (L-Trp) on growth, cerebral 5-HT concentration, stress hormone concentration, oxidative stress status, and behavior response of pigs under social stress, and also to determine an optimal daily total Trp intake that would benefit nursery pigs under social stress. Seventy two individually housed barrows at 6 wk of age were randomly allotted to 6 treatments with supplementation of 0, 2, 4, 6, 8, or 10 g L-Trp/kg to corn and soybean meal-based feedstuffs. Pigs were fed assigned feedstuffs for 15 d. Body weight was measured on d 0, 5, 10, and 15. Saliva and blood were collected on d 5, 10, and 15. On d 5 and 10, all 12 pigs in each treatment were paired in 6 new pens to record behavior for a 2-d period and returned to original individual pens. On d 15, pigs were euthanized to obtain hypothalamus. During the first 5 d, ADG and G:F increased (linear, P < 0.01) with increasing supplemental L-Trp. During the entire 15 d, ADG and G:F increased (linear, P = 0.01 and P < 0.01, respectively) with increasing supplemental L-Trp. Estimates of the daily total Trp intake based on ADG on d 15 were 10.8 g/d (P < 0.01; R(2) = 0.16) using a 1-slope broken-line analysis. Hypothalamic 5-HT and 5-hydroxyindoleacetic acid increased (linear, P < 0.01 and P = 0.03, respectively) with increasing supplemental L-Trp. Malonedialdehyde in plasma and hypothalamus, as well as salivary cortisol, on d 15 decreased (linear, P = 0.01, P < 0.01, and P < 0.01, respectively) with increasing supplemental L-Trp. Plasma urea nitrogen decreased (linear, P < 0.01) with increasing supplemental L-Trp. Increasing supplemental L-Trp affected pig behaviors during the first 2-d observation period by decreasing (quadratic, P = 0.04) lying, decreasing (linear, P = 0.04) sitting, and increasing (linear, P = 0.02) drinking. Overall, supplementation of L-Trp improved growth performance of 6 wk-old nursery pigs under social stress in association with increasing hypothalamic 5-HT production, reducing stress hormone concentrations, decreasing lipid peroxidation, increasing drinking, and reducing sitting and lying. Increase in BW gain of nursery pigs under social stress was maximal when daily total Trp intake was 10.8 g.