Twenty-four crossbred Florida native wether lambs, 28 kg initially, were assigned randomly to a basal diet (31 ppm Mn dry matter basis) supplemented with either 0, 500, 1,000, 2,000 or 4,000 ppm Mn from feed grade MnO or 2,000, 4,000 or 8,000 ppm Mn from reagent grade MnCO3. There were three sheep/treatment, with ad libitum access to feed and tap water. After 84 d, all animals were slaughtered and tissues removed for analysis. Dietary Mn at the highest levels from either source reduced (P less than .05) average daily feed intake and average daily gain. Hemoglobin and hematocrit were not affected by dietary Mn. Manganese concentration in kidney, spleen, heart, skeletal muscle, bone, serum (P less than .01) and liver (P less than .05) increased as dietary Mn increased. Kidney P was reduced (P less than .05) as dietary Mn increased; however, no other tissue minerals were affected. At comparable levels of supplementation, sheep fed feed-grade MnO exhibited numerically greater tissue Mn concentration than did those fed reagent-grade MnCO3 and regression analysis indicated higher rates of tissue uptake in MnO-supplemented sheep. Reduced feed intake and gain and apparent Mn homeostatic failure in liver were the only observed effects that may have been related to Mn toxicosis.