Vegetal tannins, extracted from the wood of Schinopsis lorenzii trees, was administered on the feed for growing ducklings to evaluate the effect on productive performances and nitrogen content in plasma and excreta. Ducks belonged to local strains of Muscovy duck, not selected for growth speed. Ninety 42 days old male Muscovy ducks were distributed in 9 open-air sandy ground pens (10 males/pen; density 0.56 ducks/ sqm) and fed ad libitum on a commercial growing mash feed supplemented with three levels of Quebracho-tannin (MGM-S product, manufactured by Unitan s.a.i.c.a., Buenos Aires): 0%, 1.5%, 2.5%. The experimental diets were randomly assigned to pens (30 ducks/ diet). Data on weight gain, feed conversion, plasma total protein level, excreta dry matter, slaughtering traits at 84 days of life were collected. Tannins added in the diets did not depress duck growth, and neither leg abnormalities nor mortality were observed. After six weeks of treatment weight gain and live weight were similar among groups, as well as slaughtering traits. Tannin level did not influence plasmatic total protein and excreta dry matter.