Products generated by filarial nematodes depress vascular reactivity by mechanisms involving endothelial cells. We hypothesized that comparable filarial-induced alterations might occur in lymph vessels. Experiments were designed to test the hypothesis that spontaneous contractions of bovine mesenteric lymphatics studied in vitro are altered by the filarial parasite Brugia pahangi. Rings of bovine mesenteric lymphatics were suspended in tissue baths and spontaneous contractions were evaluated for rate, rhythm, and amplitude. Rings that met inclusion criteria (rate > 1.8/min, regular rhythm, and an amplitude > 500 mg) were randomly exposed to B. pahangi or used as controls. Parasites were added to the bath at time zero. Changes in rate, rhythm, and magnitude of spontaneous contractions were evaluated every 10 min. Comparisons were made within control or Brugia-infected groups over time and between groups (B. pahangi versus controls). The presence of B. pahangi significantly depressed the frequency of spontaneous contractions when compared with controls. Control rings were stable over time, without changes in rate, rhythm, or amplitude. However, B. pahangi altered both the rate and rhythm of spontaneous contractions. Since spontaneous contractile activity is likely to be important in the propulsion of lymph, alterations of contractile activity could result in lymphedema. Thus, filarial factors may be responsible, in part, for altered lymphatic function seen in lymphedema. Pharmacologic intervention aimed toward influencing host-parasite metabolic interactions may, in this complicated scenario, prove useful.