1. Diets containing 500 g high‐glucosinolate rapeseed meal/kg or an equivalent amount of soyabean meal as the only protein supplement were fed to layer‐type chickens and two broiler strains from 1 to 56 d of age. Additional groups of the former were maintained on the diets until they were 16 and 28 d old.

2. The rapeseed meal produced thyroid hypertrophy in all strains but , reduced the growth rate of only one of the broiler strains. The livers of chickens fed on rapeseed meal were enlarged and DNA analysis indicated hyperplasia, but no macroscopic lesions were found.

3. The activities of aspartate transaminase, lactate dehydrogenase and alkaline phosphatase in the plasma were increased by rapeseed meal, suggesting liver damage.

4. In all strains, feeding rapeseed meal increased plasma total protein, albumin and cholesterol and decreased urate. Hyperglycaemia accompanied by a decrease in plasma triglycerides occurred in the layer strain.

5. Through its extra‐thyroidal toxicity ( — )5‐vinyl‐oxazolidine‐2‐thione (goitrin) was probably responsible for most of these changes.