Cholinesterase activity and responses to pressure, acetylcholine, histamine and eserine were studied in isolated intestinal loops from 15 rhesus monkeys, 4 rats and 31 guinea pigs subjected to whole body x-irradiation, and in loops from paired control animals. Ileal and jejunal nonspecific cholinesterase levels were reduced in rats 48 hours after 650 r, and in guinea pigs 48 hours after 250 r, but not in rhesus monkeys 48 hours and 7–9 days after 800 r. Monkey plasma cholinesterase levels showed only a small preterminal decrease when measured daily after 800 r. The peristaltic responses of monkey and guinea pig intestinal loops to intraluminal pressure were normal at the above intervals after radiation. In monkeys, rats and guinea pigs the dose of acetylcholine required to elicit a contraction was unchanged by radiation regardless of whether threshold dose or the dose which produced 50% of the calculated maximal response was measured. The height of the maximum contractile response of monkey ileal and jejunal loops to acetylcholine was greater at both time intervals after radiation than in control loops. This difference was not found in rats or guinea pigs or in monkeys after histamine or eserine. These observations cast doubt on the idea that changes in intestinal cholinesterase activity or in the sensitivity of intestinal muscle to acetylcholine play a significant part in the vomiting and the changes in intestinal motor activity which follow x-irradiation.
