Dipodomys microps is a dominant rodent species in the Study Areas. Table 1 presents Pu and Am concentrations in tissues of D. mtcrops from five Study In Area 5, NTS, where D. microps and D. merriamt are co-dominant, Areas. these two similar granivore species are pooled. In general, Dipodomys had relatively low Pu tissue burdens. In addition, tissue burdens and their Pu/Am ratios exhibited high variability, especially in carcasses, relative to values reported in both soil In each Study Area, the mean of at least one Ditpodomys tract, or carcass, was significantly different (P<.01) soil or vegetation ratios. These differences are less and vegetation samples. tissue ratio, pelt, GI from one or both mean pronounced in Area 1i-c. In respect to Pu and Am tissue burdens, the only consistent trend evident was that carcass burdens were much lower than pelt or GI tract burdens; otherwise, either pelt or GI tract had the highest concentrations depending on the Study Area examined. The highest tissue burdens for D. microps were from Clean We believe this is due to biased representation of samples from Slate 2, TTR. Data on the relationships between centers of animal activity, Pu near GZ. activity strata, and Pu tissue burdens are discussed later in this report. Differences between levels of Pu and Am and their ratios in tissues is expected between samples from distinct event sites, principally because of dissimilar concentrations of original source materials used in the respective events. The high variability in Dipodomys tissue samples and the differences between Pu/Am ratios in animal, vegetation, and soil samples from the same Study Area are more difficult to explain but may be attributable to several factors. It has been well established that Pu and Am are not distributed homogeneously in animal tissues upon uptake by inhalation or ingestion (Wick, 1967; and others). The amount of these transuranic elements taken up by a particular organ varies due to interaction of chemical and physiological processes and upon the chemical form of the radioisotope. Thus, rates of uptake and elimination of Pu and Am may be related to the species of animal investigated, its age, general physiological condition, and the mode of radioisotope uptake (Wick, 1967; Jee, 1972; Lindenbaum and Rosenthal, 1972). In addition, it has been suggested that plants may preferentially uptake Am over Pu (Romney et al., 1975). Hence, variations in Pu and Am concentrations and their ratios in animal tissues from the same event site may be due to physiological differences between species or even individuals, age during radioisotope uptake, mode and chemical form of radioisotope uptake, and differences in food items ingested. In addition, animals are mobile to varying degrees; hence, they are exposed to a more varied transuranic environment than individual plants. Additional sources of variability may be related to laboratory techniques. Methods of preparation of samples for radioanalysis vary between soil, vegetation, and animals. Counting errors and tracer yields are less consistent in tissue samples than soil samples (Major, Pers. comm.). This source of varia- bility may be due to interfering reactions or to the process of homogenizing specimens and obtaining subsamples of homogenate. Difficulties in obtaining subsamples of homogenates from animal samples, where elements may be concen- trated in particular tissues, have been reported in the literature (Sturges et al., 1974). 198