A mean population of 20 cotton rats inhabited the banks of a small radioactive liquid waste pond (=0.39 ha) in Tennessee during the summer of 1977. Radiocesium concentrations in common shoreline plants (Eleocharis, Juncus, Typha, and Polygonum) ranged from 80 pCi/dry g in Juncus to 35,800 pCi/dry g in Eleocharis. The mean (±S.E.) 137Cs concentration in cotton rat GI tracts was 2283 (±591) pCi/dry g (N = 14). The mean (±S.E.) whole-body burden of 137Cs in 14 rats sampled from June to September was 44467 (±13,142) pCi. Mean 137Cs body burdens in cotton rats increased from 32 pCi/g live weight in May to 208 pCi/g live weight in August and declined to 3 pCi/g live weight in December. The mean (±S.D.I percent distribution of the whole-body contents among pelt, GI tract, and carcass was 12 (±3), 28 (±12), and 60 (±9), respectively. The calculated mean (±S.E.) ingestion rate of 137Cs, assuming rats recaptured on the pond's banks for longer than 42 days were at equilibrium, was 1792 (±504) pCi/day. The concentration of 137Cs in shoreline plants, rat GI tracts, and rat bodies indicated that cotton rats, which are herbivores, accumulated their body burdens by foraging along the contamination zone bordering the pond shoreline. A maximum mean estimate of the amount of 137Cs annually exported by cotton rats from the pond is 8719 nCi or =10−6% of the total amount estimated to be present in the pond's sediments.