Observations were made for the third grazing season (April through September of 1961) of the effect of management systems, including medication, on the development of internal parasitism in lambs at Beltsville, Maryland. The systems included the feeding of soilage to lambs in dry lot (Band 1), transfer of lambs to "clean" pastures at biweekly intervals (Band 2), and rotation among separate parasite-contaminated pastures at approximately biweekly intervals (Bands 3 and 4). The soilage was cut daily from areas that had not been grazed since the pastures were plowed and reseeded. The soilage was fed only to the lambs. The feeders were placed behind a creep so that the ewes could neither consume any of the soilage nor contaminate the forage with feces. National Formulary (N.F.) phenothiazine (avg particle size, 7.0 jt) was used for low-level and therapeutic dosing of animals in Band 3 until 7 July, and purified phenothiazine (avg particle size 6.9 A/) was used thereafter. Only the purified product was used for the lambs of Bands 1, 2, and 4. Antemortem data, including hematocrit determinations and parasite egg counts, were obtained from 20 lambs of each band. Worm counts, obtained at necropsy from 10 lambs of each band, showed that lambs in Band 1 (zero-grazing) remained essentially worm-free throughout the period of observation. Effective control of haemonchosis was also obtained in Band 2 by combining "clean" pasture grazing, monthly therapeutic dosing with purified phenothiazine, and low-level purified phenothiazine in a free-choice mineral mixture. Weight gains of lambs in Bands 1 and 2 were significantly greater than those of lambs in Bands 3 and 4 up to weaning. The N.F. phenothiazine, given free-choice and in therapeutic drenches, failed to control haemonchosis in Band 3. Clinical signs developed early in these lambs, despite doubling the therapeutic dose on 7 July. Better control of Haemonchus contortus infections was achieved after changing to purified phenothiazine on 14 July. Lambs in Band 4 developed haemonchosis at a later date than those in Band 3, even though they had received about 50 g less phenothiazine (purified only) per lamb in therapeutic drenches. During the treatment phase of the investigation, PCV in bands 1 and 2 were significantly higher than that in Bands 3 and 4, and were closely related to the number of parasites found in the different bands. Weight gain of lambs in Band 2 were significantly greater than that of lambs in Bands 1, 3, and 4 during phase 2. The weight gain of Band 1 lambs was directly influenced by the feeding program. Under the conditions of these experiments, purified phenothiazine was somewhat superior to N.F. phenothiazine for controlling Haemonchus infections, although neither product controlled clinical haemonchosis satisfactorily. Results of studies conducted at the Agricultural Research Center, Beltsville, Maryland, during 1959 and 1960 (Lindahl et al., 1963; Colglazier et al., 1968) confirmed published reports by other investigators (Hinds et al., 1961; Tiwari et al., 1961) that internal paraReceived for publication 5 May 1970. * Animal Science Research Division, ARS, Beltsville, Maryland. t National Animal Parasite Laboratory, Veterinary Sciences Research Division, ARS, Beltsville, Maryland. + Present address: Division of Research Grants, NIH, Bethesda, Maryland. ? Present address: Food and Drug Administration, BSI, DPT, PRB, Washington, D. C. 11 Biometrical Services, ARS, Beltsville, Maryland. sitism could be controlled by raising lambs in dry lot; that the average level of parasitism acquired by the lambs was related to the degree of exposure to infective larvae (Gordon and Turner, 1946; Spedding, 1954, 1955, 1956; Gibson, 1956; Levine et al., 1956, 1958, 1960; Reinecke and Thomas, 1959; Michel, 1964; Zimmerman, 1965); and that therapeutic treatment with N.F. phenothiazine did not result in effective control of Haemonchus contortus (Drudge et al., 1957a, b; Leland et al., 1957; Levine and Garrigus, 1961; Colglazier et al., 1967). Raising lambs in dry lot has been successful in controlling internal parasitism, but the use of large quantities of concentrates and dry roughages are required. It seemed desirable to