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This study was conducted to compare the availability of Zn from two Zn sources, an amino acid chelate and ZnSO₄. In three experiments, 78 Zn depleted and 24 Zn adequate pigs were used. Pigs were depleted of Zn by feeding an isolated soy protein, semi purified diet containing 17 ppm Zn. Of the 78 depleted pigs, 60 pigs were Zn repleted. During Zn repletion in Exp. 1, depleted pigs were fed the low Zn diet supplemented with 5, 15, or 45 ppm Zn either as ZnSO₄ or as Zn amino acid chelate (ZnAAC). In Exp. 2 and 3, low Zn diets were only supplemented with 45 ppm Zn. Zinc adequate pigs, used in Exp. 1 and 2, were fed the 45 ppm supplemental Zn diets. To evaluate differences in site and rate of apparent Zn absorption, chromic oxide was added to the diets of depleted pigs in Exp. 1 and 3. In all experiments, a 24-d period was sufficient to severely deplete the porcine body Zn stores, and to cause parakeratosis and growth retardation. Serum Zn concentrations and serum ALP-activities of depleted pigs dramatically decreased (P < .01) during the first 14 d of Zn depletion. At the end of Zn depletion, Zn contents in liver, kidney, pancreas, brain, and small intestine tissues of pigs fed the low Zn diet were reduced (P < .01) by 10 to 40 % compared with the adequate pigs fed the ZnSO₄ and ZnAAC diets. In Exp. 2, the growth retardation was associated with a low (P < .05) serum mitogenic activity and pituitary RNA content of depleted pigs compared with pair-fed adequate pigs. Moreover, the growth hormone mRNA fraction tended to be reduced (P < .10) for the Zn depleted pigs. In Exp. 1, the apparent absorption of Zn was higher (P < .01) for pigs fed ZnAAC compared with the ZnSO₄ group; however, this was not confirmed in Exp. 3 unless coefficients were corrected for Cr recovery. Furthermore, absorption of Zn occurred primarily within jejunal and distal segments of the small intestine. In the balance of Exp. 3, disappearance rates of Zn, Cu, Fe and DM were higher (P < .01) in depleted pigs fed ZnAAC compared with ZnSO₄. The recovery of Cr also was different (P < .01) between pigs fed the ZnSO₄ (87 %) and ZnAAC (70 %) diets. Moreover, the moisture content of the fecal matter was 11 % higher (P < .01) for the ZnAAC group compared with pigs fed ZnSO₄. In Exp. 1, depleted pigs fed the 15 ppm ZnSO₄ and ZnAAC diets regained their ability to grow, however, replenishment of body fluid and tissue Zn pools did not occur within the 24-d Zn repletion period. Both the 5 ppm ZnSO₄ and ZnAAC groups did not respond to Zn repletion within a 12-d period. In all experiments, the rate and degree of repletion of body fluid and tissue Zn stores was not different between pigs fed the 45 ppm ZnSOq and ZnAAC diets, although a higher (P < .05) serum mitogenic activity was observed for the adequate pigs fed ZnAAC compared with ZnSO4. In conclusion, an amino acid chelate did not improve growth, or rate and degree of replenishment of body fluid and tissue levels of Zn compared with pigs fed ZnSO₄. However, ZnAAC may have influenced intestinal luminal conditions since a higher rate of disappearance of Zn, Cu, Fe, Cr, and DM was measured. / Ph. D.