Radioactive iodine-131 as both as free iodide ((NaI)-I-131) and covalently bound to aniline (aniline-I-131) was added to the drinking water of two Leghorn laying hens as a single dose and also as a cumulative dose over 1 week. The radioactivity of the principal parts of the eggs, i.e. shell, white, and yolk, was measured, and the radioactivity levels per gram material, and percent of the total radioactivity were calculated. The radioactivity measurements were continued for 1 month following the administration of I-131. In the case of the single dose administration, the results obtained showed that about 15% of the total radioactivity administered as (NaI)-I-131 was transported into the egg structure; compared to only about 1% for aniline-I-131. After cumulative administration, about 15% of the total administered radioactivity was transported into the egg structure with both forms of I-131. This was probably because of metabolic cleavage of iodine bonds in the labeled aniline molecules during the longer period of exposure. These results also showed considerable accumulation of I-131 in the egg yolks. In the case of the single dose administration, I-131 can be detected in eggs up to about 20 days after administration, and up to about 30 days, in the case of the cumulative administration over 1 week. (C) 2003 Elsevier Science Ltd. All rights reserved.