Both sexes of Moniliformis and Macracanthorhynchus concentrate methionine against a chemical difference. The uptake of methionine by both sexes of Moniliformis is inhibited by alanine, serine, leucine, and isoleucine-these inhibitions are reciprocal. The uptake of serine and alanine is not affected by methionine in female Macracanthorhynchus; however, methionine does inhibit the accumulation of those amino acids in males of Macracanthorhynchus. The effect of a mixture of four neutral amino acids on the uptake of methionine can be predicted by the generalized equation derived by Read et al. (1963, Ann. N. Y. Acad. Sci. 113 Art. 1: 154-205). Considerable information is available concerning the entrance of potential nutritional substances into cestodes (see the review by Read and Simmons, 1963, as well as Read, Rothman, and Simmons, 1963); however, there is a paucity of such data pertaining to that process in the Acanthocephala (Laurie, 1957, 1959). Like the cestodes, Acanthocephala have no gut and it is assumed that nutrients enter through the body wall. Read et al. presented exhaustive data concerning the initial rates of entry of a number of amino acids into Hymenolepis diminuta. In that study they demonstrated that amino acids were concentrated against a chemical gradient and that there was competitive inhibition between certain groups of amino acids. Those authors further demonstrated that there was inhibition of uptake of a given amino acid in the presence of a complex mixture of amino acids and that this competition could be predicted by an extension of the Lineweaver-Burk treatment of enzyme kinetics. The following study was undertaken: (1) to obtain information concerning the permeation of a selected group of neutral amino acids into two species of acanthocephalans and (2) to Received for publication 20 March 1964. * This research was supported in part by U. S. Public Health Service Grants AI-01384 and 2E-106. t Present address: California State College, Fullerton, California. further test the application of the generalized equation of Read et al. (vide supra). MATERIALS AND METHODS Acanthocephala used in this study were obtained from two sources. Moniliformis dubius were reared in male Sprague-Dawley rats (Holtzman Rat Co.). Cystacanths of Moniliformis were obtained from laboratory-reared and infected Periplaneta americana. Rats weighing 70 to 90 g were infected with 25 12-week-old cystacanths. The hosts were maintained in groups of ten in 24by 18by 12-inch wire cages in air-conditioned quarters: Food and water were provided ad lib. until the time of autopsy at 5 weeks. Rats were killed by a blow on the head and the small intestine removed to a container of saline composed of NaCl 120 mM, KCI 4.8 mM, CaCl2 2.6 mM, MgSO4 1.2 mM, and Tris (hydroxymethyl) aminomethane: maleate buffer 25 mM at pH 7.4. The intestine was opened with scissors and the worms gently removed from the mucosa. Macracanthorhynchus hirudinaceus were carefully dissected from swine small intestines at the Houston Packing Company and transported to the laboratory in the above saline in thermos containers at 36 to 39 C. Both species of parasites were separated according to sex and treated separately throughout the remainder of the experiments. The worms were preincubated in 30-ml beakers in the above saline for 60 min at 37.5 C. Following the preincubation period the worms were blotted and incubated in 30-ml beakers containing the amino acid solution (10.0 ml for Macracanthorhynchus, 5.0 ml for Moniliformis) for 3 min. The various C14 amino acids were used at a specific activity of 0.6 ,uc/,mole. After 3 min (-+5 sec) incubation, the worms were removed, rinsed rapidly in three changes of the saline (50 ml each), blotted, and placed in tubes containing a