In two series of experiments plasma thyroxine and triiodothyronine concentration were examined in 56 adult hens and 139 meat type chickens of Biała Brwinowska strain fed up to 7 weeks of age a diet containing "OO" rape seeds, wheat and barley (diet B) or maize, soyabean meal and wheat (diet C). Relationship between reproductive performance of adult hens and plasma thyroid hormone concentration was assessed. At the age of 7 weeks, chickens fed diet B had significantly lower body weight (except of males in 1994) and, despite of its low (0.7-0.9 µmol/g) glucosinolate content, significantly higher plasma thyroxine concentration than control birds. No significant diet-related difference was found in their plasma T₃ level and T₃/T₄ ratio. Also no differences were found in reproductive perfomance of one year old hens related to the diet fed in the first 7 weeks of life. In addition, in both series of experiments plasma T₄ concentration was positively correlated with egg fertility (P≤0.05) while T₃, negatively to the hatchability rate (P≤0.01).