MATERIALS AND METHODS Cerium-144 (III) chloride was obtained from the Oak Ridge National Laboratory in a 1 M HCl solution. The isotope as received contained no other y-emitting impurities. Dilutions were made with distilled water to obtain a dose solution which was 0.2 M in HCl and contained 0.5 uci/ml +4*cecl3. Adult male BALB/c mice 5 to 6 months old were maintained on a diet of Purina laboratory chow with water ad Ztbttwn throughout the course of the experiment. Three groups of mice were used, one for each of the three fasting-feeding regimens studies. In each group, three mice were killed at 1, 2, 4, 8, 12, 16, or 24 hours after dosing. The experiment thus involved a total of 63 mice with 21 in each group. These were: I, fasted for 24 hours, dosed, fasted thereafter; II, fasted for 24 hours, dosed, fasted for 8 hours, then given feed ad libitum; III, fasted for 24 hours, given feed ad libitum for 4 hours before dosing, dosed, fasted again. The mice were dosed by gavage with 0.1 uCi of 144Cecl; (0.5 pCi/ml) in 0.2 ml of 0.2 M HCl solution. After dosing was completed, the mice were returned to cages which were fitted with wire mesh bottoms to inhibit ingestion of contaminated excreta. The mice were asphyxiated with CO», the digestive tracts excised and sectioned (Figure 1), and the sections placed into gamma counting tubes for !*4ce analysis. Figures 2 and 3 show the averages for three mice expressed as percent of the administered dose. The administered dose standards and the intestinal sections were counted in equivalent volumes of water under integral counting conditions in an automatic gamma cowmter. The 144ce activities of the gut sections were counted in tubes that were filled to a 3-m]l total volume with water to minimize the effect of changes in sample volume on the count rate of the instrument. RESULTS Results of the experiment are shown in Figures 2 and 3. The intestines of groups I and II were essentially clear of !*4*ce activity of the gut 12 hours after dosing. The group allowed to eat before dosing and not thereafter had G.I. tracts containing dissection, and the !**ce activity pattern of this group of in all parts 4 hours food on mice exhibi- ted an anomalous rise at later hours. In addition, the gut transit time of group III was about 12 hours longer than that of groups I and II. Results obtained in fasted mice with cerium are indicative of rapid transit of a soluble metal species along the empty gut with essentially 638