Syphacia sp. is a common intestinal parasite in conventionally-housed laboratory rodents. Although gross lesions are rare in oxyuriasis, it is possible that more subtle changes may develop, which may affect research results. In this study, we analysed the responsiveness to beta-adrenergic stimulation by isoproterenol (ISO) of left atria isolated from Syphacia-infested (SYPH) and control, non-infested adult male Wistar rats (CONT). In the non-infested animals, ISO pD(2) was not significantly changed by ivermectin treatment. Whereas the maximal inotropic response to ISO was not significantly affected, the pD(2) value was decreased in SYPH (7.61 +/- 0.09, n = 7, vs 8.21 +/- 0.25 in CONT, n = 5, P < 0.05), indicating lower sensitivity to beta-adrenergic stimulation. This change was similar to that caused by a classic stressor, namely repeated immobilization, in non-infested rats (IMMO). In this group, ISO pD(2) was 7.62 +/- 0.14, n = 6 (P < 0.05 with relation to CONT). The results indicate that infestation with Syphacia sp. is as effective as immobilization at diminishing cardiac reactivity to beta-adrenergic stimulation. It is thus possible that oxyuriasis may affect the response of other tissues to physiological modulators.3716367