This study was to evaluate the effect of oral Trp supplementation to nursing piglets on serotonin (5-HT) metabolism, growth performance, and behavior during nursing (from 14 to 21 d of age) and nursery (from 21 to 56 d of age) periods under social mixing stress at weaning (from 21 to 22 d of age). Eighty piglets from 10 sows (4 males and 4 females per litter) at 14 d of age were randomly allotted to 2 treatments based on sex. Treatments (Trp, 0.46 g/kg BW/d; or Ala + glucose, 0.38 + 0.42 g/kg BW/d, respectively) were orally given to piglets during nursing period. At weaning on d 21, 6 piglets from each treatment were euthanized to collect hypothalamus to analyze 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) concentrations. Remaining 68 piglets were weaned and moved to nursery pens. Pigs within a treatment, same sex, but from different litters were paired and housed together (2 pigs/pen, 17 pens/treatment). Normal and aggressive behaviors were recorded from 21 to 22 d of age immediately after weaning and mixing. All pigs were fed the same diet during nursery period. Growth performance was measured during nursing and nursery periods. Data were analyzed using Proc Mixed of SAS with treatment and sex as fixed effects. Oral Trp feeding during nursing period increased (P ≤ 0.05) hypothalamic concentration of 5-HT and 5-HIAA (2.84 to 5.94 and 59.1 to 104.6 ng/mg of protein, respectively), tended to increase the frequency of visits to the feeders (P = 0.057, 0.42 to 0.83%) during mixing period and G:F ratio (P = 0.094, 0.440 to 0.514) during the first week of nursery period. Collectively, oral Trp feeding during nursing period increased hypothalamic serotonin affecting eating behavior and potentially enhancing feed efficiency immediately after weaning.