Abstract Several N-bromoalkylderivatives of triiodothyronine (T 3) and thyronine (T 0) were synthesized and analyzed for their ability to bind to the nuclear receptor of T 3 in competition experiments with 125I-L-T 3 using either the crude nuclear extract or a partially purified preparation of the T 3 receptor. N-bromoacetyl-L-T 3 was the most potent with K d varying between 10 and 100 nM within nuclear material preparations, higher values being found in crude nuclear extract. N-bromoacetyl-DL-T 0 was not bound. Incubating the partially purified nuclear T 3 receptor with N-bromoacetyl-L-T 3 led to a significant loss of T 3 binding site concentration (up to 32 %) without significantly altering the affinity for T 3, while N-bromoacetyl-T 0 was ineffective. This suggests that a specific covalent interaction could occur between N-bromoacetyl T 3 and the T 3 binding site region of the nuclear receptor.