Two experiments were conducted, using typical commercial swine production conditions, to determine effects of dietary arginine supplementation during early gestation on the performance of gilts and sows. In Experiment 1, 62 Landrace gilts and 113 sows were used to evaluate the effect of dietary arginine supplementation during early gestation (d 1 to 30) on gilt and sow performance. On d 1 of gestation, all gilts and sows were randomly assigned to a corn-soybean meal basal diet supplemented with either 1.3% l-arginine-HCl or 2.2% l-alanine (isonitrogenous control). The total number of piglets born per litter for arginine-supplemented was increased (P < 0.01), compared with the control group. In Experiment 2, 155 Landrace sows were used to evaluate the effect of dietary arginine supplementation between d 1 and 14, d 15 and 30 or d 1 and 30 on sow performance. Among these sows, 24 with an average parity of 2.54 were used to determine the effect of dietary arginine supplementation on plasma amino acid composition and the concentration of plasma metabolites. The total number of piglets born per litter for arginine-supplemented sows tended to increase (P = 0.08), compared with the control group, with the greatest improvement in sows treated from d 1 to 14. The concentrations of nitric-oxide metabolites, arginine, proline, ornithine and spermidine were increased (P < 0.05) in the plasma of arginine-supplemented sows on d 14 and 28 of gestation, as were agmatine and putrescine on d 14 of gestation. The concentrations of urea and glutamine were lower in the plasma of arginine-supplemented than in the control group. These results indicate that dietary arginine supplementation in early gestation improved the reproductive performance and plasma biochemical metabolites in sows.