Although pharmaceuticals are increasingly found in surface waters, environmental levels of many of these compounds are not acutely toxic to model test organisms. Prior to conducting appropriate ecological risk assessments of pharmaceuticals, the mode of action-based biomarkers needs to be developed for non-target species. To evaluate toxicity of the beta-adrenergic blockers propranolol and metoprolol on non-target biota, we assessed standard ecotoxicological endpoints after 9 days of subchronic exposure in a transgenerational study with Daphnia magna. On day 9 of exposure, the lowest observed effects concentration (LOEC) for growth were 0.44 mg L(-1) for propranolol and 12 mg L(-1) for metoprolol. Total fecundity LOECs for the 9-day exposure were 0.11 mg L(-1) and 6 mg L(-1) for propranolol and metoprolol, respectively. In addition to typical ecotoxicological endpoints, we evaluated the utility of employing the physiological biomarkers of heart rate and metabolic rate following subchronic and acute exposures to propranolol and metoprolol. Propranolol and metoprolol LOECs for heart rate were 0.055 mg L(-1) and 3.1 mg L(-1), significantly lower than the LOECs for growth and reproduction. At these concentrations, heart rate was significantly lower than the control values for both compounds. Daphnia magna endpoint sensitivity to chronic propranolol and metoprolol exposure was mortality < growth < fecundity < heart rate. Second generation D. magna were less sensitive to both compounds. Responses of the physiological biomarkers heart rate and metabolic activity suggest that propranolol and metoprolol exerts sublethal toxicity to D. magna at lower concentrations than observed in the classical endpoints.