Concentrations of 129I and values of the 129I/127I ratio are reported in one sample of indigenous vegetation and in over forty additional individual thyroids of man, wild deer and beef cattle in Missouri. The results of this and other studies in our laboratory indicate the following order for successively lower values of 129I/127I ratios in the local environment: Rain, wild deer, commercial milk, beef cattle and human. The value of the 129I/127I ratio in the single vegetation sample is intermediate to the mean values in wild deer and commercial milk, but well within the range of values observed in both. These results are consistent with a geochemical cycle in which iodine that is enriched in 129I is transported via air into the central U.S. and then diluted with other iodine-especially mineral iodine that is added to the diets of domesticated animals-as the iodine deposited from air moves through the local ecosystem. Differences in the diets of beef and dairy cattle or differences in the biological life-times of iodine in thyroids and mammae, and hence the degree of equilibration with body iodine, may explain the lower value of the 129I/127I ratio in beef thyroids than in milk.