The RNA per unit protein of trichinella-infected diaphragm muscle increased as the number of muscle larvae present increased and it was shown that there is a minimum number of larvae required for detection of the full pattern of change known to occur in the RNA per unit protein of infected muscle. When methyridine was used to abbreviate the stay of adults in the intestines of mice the full pattern of change in RNA per unit protein of infected diaphragms was discernible only when the drug was given after day 9 PI. The total protein of infected diaphragm muscle dropped below that of controls about day 15 PI and remained below that of uninfected diaphragms throughout 60 days of infection. Uridine incorporation and RNA per unit protein of liver tissue from trichinosed and uninfected mice was similar throughout 40 days. Whole cardiac muscle from trichinella-infected animals showed no difference in RNA per unit protein from that of control mouse heart muscle over 40 days of infection. Methyridine was shown to have no influence over the RNA per unit protein of mouse diaphragm muscle. When RNA per unit protein of infected diaphragm muscle was related to wet weight, dry weight, or protein a similar magnitude and pattern of change in RNA per unit protein was observed over a 40-day period. An hypothesis of events occurring after entrance of trichinella larvae into host muscle fibers is presented. Trichinella-infected muscle fibers have been shown to undergo marked morphological changes (Fasske and Themann, 1961; RibasMujal and Rivera-Pomar, 1968) and enzymatic alterations (Maier and Zaiman, 1966). These changes are accompanied by dramatic alterations in the metabolism of protein, RNA, and DNA (Stewart and Read, 1972a, b, 1973). The present study reports changes in: (1) RNA per unit protein of trichinosed diaphragm muscle under different conditions of infection; (2) RNA per unit protein or RNA metabolism in tissues other than skeletal muscle of trichinella-infected mice; and (3) total protein of trichinosed diaphragm muscle. In addition, the effects of the anthelminthic methyridine on RNA per unit protein of uninfected mouse muscle was investigated. MATERIALS AND METHODS The source of trichinella, method of excystment of muscle larvae, and procedure for the infection of experimental animals were those used in previous studies (Stewart and Read, 1972a, b). Male 6-week-old Swiss white mice (Texas Inbred Mice Co., Houston, Texas) were used in all experiments. Unless otherwise indicated, methyridine was given on day 11 postinfection (PI) to remove the adults from the intestines of experimental animals Received for publication 12 December 1972. * This work was supported in part by a grant from the NIH, U. S. Public Health Service (5 T01 AI00106). (Stewart and Read, 1972a). All experimental mice were infected with 1,000 larvae unless otherwise stated. A modified method of Schmidt and Thannhauser (1945) was used in all experiments in which the RNA and protein fractions were removed from muscle. RNA and protein were assayed by methods used in previous studies (Stewart and Read, 1972a). In an experiment dealing with changes in RNA per unit protein and rates of RNA synthesis in liver of infected versus uninfected mice, animals were injected intraperitoneally with 15 uCi of 3H-uridine (sp. act. = 6.4 Ci/mM) and killed 24 hr later. A piece of the left lobe of the liver was removed from each mouse, rinsed twice in KRT, blotted on filter paper, and placed in a dry test tube immersed in a dry ice-acetone bath. The tissue was homogenized in 2 ml of cold 0.5 N perchloric acid and submitted to the SchmidtThannhauser extraction procedure. The radioactivity of 2to 0.5-ml aliquots of the RNA fraction of the liver tissue of each of these mice was determined in a Packard Tri-Carb Scintillation Spectrometer. The tissues in experiments involving diaphragm muscles and cardiac muscle were treated in the same manner as described above, except that no radioisotopes were used. In experiments in which the dry weight of muscle was determined, the diaphragms of mice were removed, rinsed once in KRT, blotted dry, placed in a 90 C oven for 48 hr at the end of which time the tissue was weighed. In all experiments 3 infected and 3 uninfected mice were killed on the days indicated in context. 3H-uridine was obtained from Amersham/Searle Corp., Houston, Texas. All other chemicals were of reagent grade. Student's t test was used to evaluate the significance of differences.