Technical Abstract: Fumonisin mycotoxins are found in corn and corn-based foods, but the effect of cooking on fumonisin toxicity has not been studied extensively. Rat feeding studies were used as an in vivo bioassay to compare the toxicity of extrusion cooked and nixtamalized (alkaline cooked) fumonisin-contaminated products to their uncooked starting materials. Extrusion reduced toxicity of one of three materials tested. The amount of reduction depended on the materials' initial fumonisin concentrations, the type (fumonisin spiked or fermented) of material, and whether or not glucose was added to the recipe. The effectiveness of nixtamalization for reducing toxicity was enhanced when ground corn was added to the recipe, suggesting that some fumonisins were sequestered by matrix binding during cooking. The findings indicate that extrusion and nixtamalization can reduce fumonisin toxicity. Further investigations to establish the conditions for obtaining optimal reductions and determine the chemical fate of fumonisins during cooking are warranted.