Lactating cows were orally dosed with I2, CH3I, NaI and NaIO3 labeled with radioiodine. For these 4 chemical forms, the effective half life in milk was approximately 1 day for the first 5 days. The peak thyroid concentration occurred approximately 2.8 days after ingestion in all the cows. Ion-exchange analysis indicated that, regardless of the chemical form administered, 97% of the iodine in the milk was inorganic. These results suggest that in dairy cows there are no significant differences in the thyroid uptake and milk transfer among the compounds tested.