To investigate the possible side effects of epinephrine on the aqueous outflow pathway, we studied the response of human trabecular cells in vitro exposed daily to the drug at various concentrations. Epinephrine at 10(-6) M caused abnormal cytokinesis and cell retraction, inhibited mitosis and phagocytosis, and induced a 4- to 5-fold increase in cAMP. After 7-10 days of exposure, notable degenerative changes were observed in the trabecular cells. Within certain limits, these effects were reversible when the drug was withdrawn. Exposure to epinephrine at 10(-5) M caused cell death within 96 hours. When a 10(-7) M concentration of epinephrine was administered for up to 10 days, no degenerative changes were seen, but mitotic activity was reduced somewhat. Pretreatment with timolol reduced, but did not completely eliminate, the cytotoxic effects of epinephrine. Cells with loose contact to the substrate were affected most. The morphologic changes and the observed loss of intracellular actin filaments indicate that the deleterious effects of epinephrine are probably mediated through damage to cellular contractile proteins. Although the precise cytotoxic action of epinephrine in vivo remains to be established, our results for primary cell cultures indicate a toxic action of this drug at concentrations of 10(-6) M or higher on human trabecular cells and suggest that, for prolonged use, the maximal dose of this drug at target sites should be in the region of 10(-7) M.