Hymenolepis diminuta-associated alterations in rat intestinal myoelectric patterns are abolished following therapeutic administration of the anthelmintic praziquantel (PZQ). Host intestinal smooth muscle myoelectric patterns, reflecting smooth muscle contractility and intestinal phasic motility, were recorded using in vivo serosal electrodes, surgically implanted on the duodenum, jejunum, and ileum. Repeated electromyographic recording from unrestrained and unanesthetized rats began 5 days after electrode implantation surgery. Three initial control recordings from each rat confirmed the appearance of normal intestinal myoelectric patterns, characterized by the interdigestive migrating myoelectric complex (MMC). All animals were subsequently infected with H. diminuta and myoelectric recordings beginning after day 8 postinfection confirmed the appearance of diminished frequency of the MMC and 2 nonmigrating myoelectric patterns, i.e., repetitive bursts of action potentials and sustained spike potentials. PZQ was used to remove the tapeworms from rats 12 days after Hymenolepis diminuta infection, as intestinal myoelectric changes become maximal at this time in tapeworm-infected rats. PZQ administered to uninfected rats at either of 2 dose levels did not affect host interdigestive myoelectric activity. After removal of the parasite with PZQ, electromyographic recordings indicated a return to normal uninfected electrical patterns within 24 hr of drug treatment. We have demonstrated that the presence of Hymenolepis diminuta is necessary to induce and maintain abnormal intestinal myoelectric patterns. The altered motor properties of tapeworm-infected rat intestine and the rapid reconversion to preinfection myoelectric patterns provides a new and unique model to examine the regulatory mechanisms of intestinal motility and its control by luminal parasites.