Mycotoxins are secondary fungal metabolites with varying chemical structures and biological effects. Contamination of foods and feeds with mycotoxins is a significant problem worldwide. The major mycotoxin producing fungi are ' Aspergillus, Penicillium' and 'Fusarium'. Occurrence of 'Fusarium' mycotoxins is common in temperate regions of the world including Canada. There is a lack of information on the effects of feeding low concentrations of feed-borne 'Fusarium' mycotoxins on turkeys. Experiments were conducted to study the effects of feeding grains naturally contaminated with low concentrations of 'Fusarium' mycotoxins to turkeys. The efficacy of a polymeric glucomannan mycotoxin adsorbent (GMA) in preventing any adverse effects was also evaluated. The contaminated diets were formulated by replacing control corn and wheat with contaminated corn and wheat, respectively. The feeding of contaminated grains adversely affected performance, metabolism and health of turkeys. Supplementation of contaminated diets with GMA prevented many of the adverse effects. The reduced performance was thought be due to decreased absorption of nutrients from the gut and due to behavioral changes caused by alterations in brain neurotransmission. This hypothesis was tested by measuring the effect of diet on intestinal morphology and brain neurochemistry. The feeding of contaminated grains adversely affected intestinal morphology and brain serotonergic neurotransmission. Supplementation of GMA prevented many of these adverse effects. The effects of feeding contaminated diets on intestinal immunity and cell proliferation, and histology of immune organs revealed adverse effects on intestinal lymphocyte subsets, cell proliferation in crypts and cellularity of lymphoid organs. GMA prevented the adverse effects on intestinal lymphocyte subsets. In summary, performance, metabolism, intestinal morphology and brain neurochemistry in turkeys was adversely affected by feed-borne Fusarium mycotoxins. The immune system of turkeys was sensitive to the effects of feed-borne Fusarium mycotoxins as reflected in adverse effects at both the systemic and at the intestinal levels. It was concluded that the feeding of grains naturally contaminated with Fusarium mycotoxins to turkeys should be minimised.