The uptake of L-proline in the rat cestode, Hymenolepis diminuta, follows adsorption kinetics. The transport system becomes partially saturated at 0.5 to 0.6 mM proline concentrations and fully so at 8 to 10 mM. Competition studies indicate that the efflux of proline is mediated, although it is a linear function of internal concentrations. A part of the exogenous proline pool becomes compartmentalized and is not available for efflux. Glucose does not influence the absorption of proline when present externally. However, high internal concentrations of glucose inhibit the uptake of proline and several other amino acids. In the past few years, it has been clearly demonstrated that several tapeworm species absorb amino acids by a mediated process (Daugherty, 1957; Daugherty and Foster, 1958; Read et al., 1960a; Read et al., 1960b; Read, Rothman, and Simmons, 1963; Senturia, 1964). The most extensive amino acid transport studies have been carried out with the rat tapeworm, Hymenolepis diminuta, with major emphasis on the permeability properties of this worm for the amino acid, methionine. The experiments presented here describe the nature of the influx and efflux of L-proline which is one of the main constituents of the free amino acid pool of H. diminuta (Campbell, 1963). MATERIALS AND METHODS Unless otherwise stated, the standardized methods of Read et al. (1963) were used for the uptake studies as well as the infection, maintenance, collection, and sampling of the biological materials. The host rats were slightly heavier (105 to 125 g) than those used by the above investigators. The radioactivity of the 70% ethanol extracts of the samples was assayed either by a gas-flow counter or by a scintillation counter, using absolute ethanol-toluene-liquifluor mixture in the latter case. With the use of proper standards, most of the data are expressed in terms of ethanol extracted dry weight or worm water (Read et al., 1963). The method of Troll and Lindsley (1955) was used for the chemical determination of proline. To justify the use of radioactivity as a measure of proline uptake, the ethanol extracts of worms Received for publication 22 April 1966. * This study was supported in part by a research grant (5 T1 AI 106-05) from the NIH, U. S. Public Health Service t Present address: Department of Epidemiology and Preventive Medicine, University of California, Davis. that had been incubated for 60 min in 1.5 uc of L-proline-4C (specific activity 0.006 mg/Auc) were chromatographed and radioautographs developed. Schleicher and Schuell 589 Blue Label chromatography paper was used with the following solvent systems (Block et al., 1958): (1) phenol:water (68:32 w/w), (2) pyridine:water (80:20 v/v), (3) butanol: formic acid:water (75:15:10 v/v), and (4) butanol:acetic acid:water (8:20:20 v/v). The test papers were exposed to Ansco nonscreen x-ray film for 5 weeks. In all solvent systems a single radioactive spot with the same rf value as the control radioactive proline was detected. The radioactive spots were sprayed with 0.2% isatin in acetone for further identification as proline. Uniformly labeled L-proline-4C, purchased from New England Nuclear Corporation, was used throughout this study. All other stable and 'Clabeled amino acids were obtained from California Corporation for Biochemical Research. Except for DL-hydroxyproline-1C, all the amino acids used were L-isomers. Other experimental details are described in context.