THE EFFECT OF FASTING ON THE TRANSIT TIME oF !44cR IN THE MOUSE GUT J. F. Weiss and H. E. Walburg Comparative Animal Research Laboratory Oak Ridge, Tennessee ABSTRACT Our work with G.I. absorption of actinide elements indicates greater absorption by fasted animals than by animals on regular diets (Weiss and Walburg, Undated). Residence time of a metallic compound in the gut may be an important factor influencing G.I. absorption. Cerium-144 (IIT) chloride was administered by gavage to fasted mice and to mice on regular feed. The G.I. tract was excised, cut into sections, and the activity of each section determined as a function of time after dosing. Our results indicate rapid transit of 144 CeCl; along the empty mouse gut. One hour after dosing, about half the Ce is in the cecal contents; about 40% remains in stomach contents. Twelve hours after dosing, only about 2% remains in the cecum; by 16 hours, almost the entire dose has been cleared from the intestine. Transit times in mice with stomach and intestines containing food were 12 hours longer than in fasted mice. These results lead to the conclusion that factors other than G.I. residence time determine G.I. absorption of actinides in mice. INTRODUCTION G.I. uptake is influenced by several factors: gut transit time, diet, fasting, health and age of the animal, chemical form and chemistry of the dose element (Durbin, 1973), and variability among individuals of the same species. Many of these factors can combine to yield high variability in absorption data. Large absorption differences between animal species due to inherent biochemical and physiological characteristics may be obscured by this variability. In view of the need to correlate abstruse data from many laboratories and to clarify some of the biological and chemical factors influencing G.I. absorption, knowledge of gut transit time is necessary. 637