Increased consumption and improper disposal of prescription medication, such as beta (β)-blockers, contribute to their introduction into waterways and pose threats to non-target aquatic organisms. Beta-blockers are widely prescribed for medical treatment of hypertension and arrhythmias. They prevent binding of agonists, such as catecholamines, to β-adrenoceptors. In the absence of agonist induced receptor activation, adenylate cyclase activation and increases in blood pressure are limited. With their widespread use, there has been rising concern about the impacts of β-blockers on coastal ecosystems, especially because wastewater treatment plants are not designed to eliminate these drugs from the discharge. Few studies have characterized the sublethal effects of β-blocker exposures in marine invertebrates. The aim of our research is to evaluate cellular biomarker responses of two commercially important filter-feeding marine bivalves, Eastern oysters (Crassostrea virginica) and hard clams (Mercenaria mercenaria), upon exposure to two β-blocker drugs, propranolol and metoprolol. Bivalves were obtained from Narragansett Bay (Rhode Island, USA) and acclimated in the laboratory. Following acclimation, gills and digestive gland tissues were harvested and separately exposed to concentrations ranging from 0-1000 ng/l of each drug for 24 hours. Tissues were bathed in 30 parts per thousand filtered seawater, antibiotic mix, nutrient media, and the test drug. Tissue samples were analyzed for biomarker assays including tissue damage (lysosomal membrane destabilization and lipid peroxidation), total antioxidant capacity, and activity of glutathione-s-transferase (GST) – a detoxification enzyme. Elevated tissue damage and changes in GST activities were noted in the exposed tissues at environmentally relevant concentrations. Digestive gland tissues were more responsive to the exposures than gill tissues. Differences in species sensitivities and responses to the exposures were also observed. These studies enhance our understanding of the potential impacts of prescription medication on coastal organisms, and demonstrate that filter feeders such as marine bivalves may serve as good model organisms to examine the effects of water soluble drugs. Evaluation of a suite of biomarkers allows us to better define molecular initiating events and subsequent key events that might be used to develop adverse outcome pathways (AOPs) for unintended environmental exposures to β-blockers.