100% clearance of radioactivity at 12 hours after dose administration. The variability observed among the data was largest in the group of mice which was allowed to eat before dosing (group IIT). DISCUSSION The results clearly indicate that the transit time of the soluble nonab- sorbed marker !44*CeCl3 is much more rapid in fasted mice than in those with G.I. tracts containing food. Such a large change in transit time for an element must certainly affect its absorption. Standardization of conditions for laboratory animals with respect to fasting would help reduce the variability among data from different sources in animal experiments measuring G.I. absorption. The authors believe that the anomalous rise in measured activities in the intestines of group III at later hours is in reality visible only because of the decrease in activity immediately preceding it. This decrease we feel is due to loss of counts in the integral counting mode caused by self-absorption of the 2.99 MeV !44pr g-emission in the thicker, full gut sections as opposed to the thinner, empty gut sections of groups I and II. Since gut transit time is a function of feeding regimen, we believe that better absorption data would be obtained using animals on regular feed. These data would then incorporate a normal gut transit time function for absorption of a substance and would be more realistic in the final analysis since adsorption or binding of the measured substances by dietary constituents is almost certainly an important factor in G.I. absorption for most heavy metallic elements. ACKNOWLEDGMENTS The technical assistance of Mrs. Faith Case is acknowledged. This research was supported by the Nevada Applied Ecology Group and U.S. Department of Energy Contract No. EY-76~-C-05-0242 with the University of Tennessee. 642