The toxicity of Aspergillus ochraceus contaminated wheat and various chemical forms of ochratoxin A (OA) were compared by feeding diets containing A. ochraceus contaminated wheat (3.0 micrograms OA/g diet) and 3.0 micrograms/g of purified OA in the K salt, Na salt, or organic acid form to broiler chicks from hatching to 4 weeks of age. All OA diets caused listlessness, emaciation, dehydration, and occasional diarrhea. Cumulative mortalities were 0, 13, 17, and 10% for the control, contaminated wheat, OA Na salt, OA K salt, and OA acid, respectively. Necropsies at the end of the experiment revealed pale friable livers, enlarged pale kidneys, and enlarged gall bladders in all OA treatments. Body weights and gain were significantly depressed throughout the experiment, and cumulative feed conversion ratios were significantly increased by all forms of OA. Relative kidney and liver weights were also significantly increased by all forms of OA. Serum analysis revealed significant decreases in total protein, albumin, globulin, cholesterol, and phosphorus concentrations and significant increases in uric acid concentrations in chicks fed all forms of OA. Determinations of median lethal dose (LD50) were conducted by dosing day-old chicks and recording mortality for 10 days. LD50 values were 4.41, 3.95, and 2.69 mg/kg for OA acid, Na salt, and K salt, respectively. These results indicated that the K salt of OA was more toxic than the Na salt in acute oral dosing. During the feeding study, results also suggested that chemical form of OA affected its toxicity, but after feeding 3.0 micrograms/g OA for 4 weeks, no significant differences in toxicity were caused by the various chemical forms of OA or the A. ochraceus contaminated diet.