The investigations were carried on the ducklings, which were given the aflatoxin B1 (AFB1) in a dose of 5?g /1000g of fodder. The AFB1 administration started on the second day after birds hatching and lasted for the next two or four weeks. In the blood serum of intoxicated ducklings there were determined: the thyroxine – T4 level, the alkaline phosphatase – ALP activity, the total protein level and, after the electrophoretic separation, the content of protein fractions. It was found that the two or four week-period of AFB1 administration causes decrease of thyroxine level. However, the thyroxine level in ducklings, which were given AFB1 for two weeks, was lower than in those, which were given AFB1 longer - for four weeks. It was also shown, that AFB1 administration was accompanied by the ALP activity decrease and the decrease of protein content in blood, resulting mainly from the albumins content decrease. The obtained data confirm the high toxicity of small amounts of AFB1 in ducklings, but also indicate the possibility of change of thyroid gland hormones level in the intoxicated birds.

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