Two experiments are described that test in mice the effect of cadmium oxide against T. spiralis after a comparatively brief exposure to medicated feed. Feed medicated with 0.015% CdO produced a significant reduction in the number of adult worms, as measured at both 7 and 11 days after a single infection with 200 larvae. Smaller concentrations (0.006 and 0.009%) had no demonstrable effect. Feed with 0.015% CdO given for only a 72-hr period reduced significantly the number of adult worms recovered at 9 days after an infection with 200 larvae. Medication schedules started from 48 hr prior to infection to 48 hr after infection were effective, but that started 6 days postinfection failed to produce a demonstrable effect. The most effective schedule started 48 hr prior to infection and terminated 24 hr postinfection. It produced not only a significant reduction in adults at 9 days, but a significant reduction in muscle larvae at 28 days postinfection. These results after a comparatively brief period of medication suggest that the preadult and young adult worms are more vulnerable to CdO than older adult worms, and thus encourage tests on the effect of brief intermittent schedules of medication. After the report that cadmium oxide is an effective swine ascaricide (Burch and Blair, 1955), various studies have tested its effect against Trichinella spiralis in mice. A small concentration (0.015%) in feed has a striking effect in reducing the number of adult worms in the intestine and larvae in muscle after a single infection (Larsh and Goulson, 1957), and the same concentration controls significantly the numbers of parasites after repeated infections (Larsh et al., 1958). In the latter case, both chemical action and factors associated with acquired immunity are involved in the reductions noted. A preliminary study in swine with the 0.015% concentration showed a striking effect in reducing the numbers of muscle larvae after three infections, but there was evidence of interference with normal weight gain (Larsh and Goulson, 1959). The latter prompted additional studies in mice with the hope of finding a concentration and/or schedule of feeding the chemical that would produce high-level anthelmintic effects without undesirable side effects against the host. The first of these attempts, showing the effect of various small concentrations of CdO (0.006, 0.009, and 0.012%) were reported recently (Larsh et al., 1962). The present report is Received for publication 22 June 1962. * This investigation supported in part by a research grant from the American Smelting and Refining Company, South Plainfield, New Jersey. concerned with a different approach to the same problem. All previous work has tested the ef ect of medicated feed given on a continuous basis throughout the selected periods. In view of the obvious advantages to be gained over a long period by intermittent medication, this approach was considered worthy of study. However, to determine whether such an approach is practical, it was necessary first to test the effect of the chemical given in feed over comparatively brief periods of time. Morever, it was hoped that the data would indicate whether or not the preadult worms and young adults are more vulnerable to chemical action than the mature males and females. The present report, therefore, deals with tests designed to obtain this information. With the knowledge that the standard concentration of CdO (0.015%) used in previous tests produces no demonstrable adverse side effects in mature mice, it was decided to include this concentration in both experiments to be reported. In one experiment, smaller concentrations (0.006 and 0.009% CdO) were used as well for comparison with earlier results after continuous feeding. MATERIALS AND METHODS The experimental procedures were described in an earlier report (Larsh and Kent, 1949). The medicated diet containing 0.015% cadmium oxide was prepared by mixing cadmium oxide with pulverized Purina Laboratory Chow. The cadmium oxide was obtained from Aska-Rid (1.5% CdO)