Dietary zinc supplementation, as zinc oxide nanoparticles, in weanling pig diets The purpose of this study was to evaluate the effects of dietary Zn (as ZnO nanoparticles) on performance, diarrhea occurrence, nutrient digestibility, Zn excretion in the feces, blood parameters, histology of gut epithelium, organs morphometry and intestinal bacterial count of weanling pigs compared to the use of pharmacological doses of Zn (as ZnO). One hundred and ninety-two 21d-weaned pigs (5.90±0.83 kg BW) were used in a randomized complete block design experiment with 6 treatments, 8 replications per treatment, and 4 animals per experimental unit (pen). The treatments were: negative control (NC): basal diet (based on corn, soybean meal, dried whey and dried plasma) with 100 mg Zn (as ZnO)/kg diet; positive control (PC): basal diet with 2,400 mg Zn(as conventional ZnO, 150nm)/kg diet and basal diet with 12, 24, 48 or 96 mg Zn (as ZnO-N, 70nm)/kg diet. Pigs were fed dietary treatments from 1 to 21 d feeding period followed by a common diet (same diet for all treatments) from 22 to 35 d feeding period. Diarrhea occurrence was recorded daily. Feces samples were collected to determine the digestibility of the diet and to quantify the Zn excreted. Blood samples were collected on the 19th day of the experiment for analysis of blood parameters. On the 21th day one pig per pen was slaughtered for the analyses of organs morphometry, intestinal epithelium histology and bacterial count. ANOVA and polynomial regression analysis (for levels of Zn as ZnO-N) were performed. Orthogonal contrasts were used to compare positive control with each level of Zn (as ZnO-N). No effects of Zn levels (as ZnO-N) were observed on performance, Zn excretion on feces, blood count, organs morphometry, intestinal epithelium histology and intestinal bacterial count. Increased levels of Zn (as ZnO-N) linearly increased the apparent digestibility of nutrients. It was observed a quadratic effect of Zn levels (as ZnO-N) on the frequency of diarrhea occurrence between 1 and 7 d and on the Zn plasma concentration. From 1 to 21 d of experimental period, lower daily feed intake for Zn levels (as ZnO-N) was observed compared to positive control. No differences were observed among the positive control and levels of Zn (as ZnO-N) for performance and diarrhea occurrence during the period 21 to 35 d. Lower Zn excretion in feces and lower Zn plasma concentration were observed for Zn levels (as ZnO-N) compared to the positive control. Zn supplementation, as ZnO-N, could not to replace the pharmacological dose of Zn (ZnO) to control diarrhea after weaning. The levels of Zn, as ZnO-N, did not cause toxicity of weanling pigs and reduced Zn excretion in the feces.