Acetaminophen (APAP) and bromobenzene (BrB) are reported to selectively inhibit plasma membrane (PM) but not endoplasmic reticulum (ER) Ca2+transport in rat liver (1). The ability of these hepatotoxicants to increase cytoplasmic Ca2+levels as a result of disrupted Caz+pumping was determined in cultured rat hepatocytes by monitoring the activity of glycogen phosphorylase a, a Ca2+-sensitive (via phosphorylase kinase) enzyme. Following exposure to 2.5 to 10 mM APAP for five minutes, dose-dependent increases in phosphorylase a activity were observed (58 to 190 U/g protein). Endoplasmic reticulum Ca2+pump activity was not inhibited after any dose of APAP (56 nmol Ca2+ per milligram protein per 30 minutes). Phosphorylase a activity remained elevated for 60 minutes after exposure to APAP (124 pYg protein). Following exposure to 0.5 to 2 mM BrB for five minutes, phosphorylase a activity also increased (58 to 229 Ulg protein) in a dose-related manner. Endoplasmic reticulum Ca2+ pump activity was inhibited after BrB exposure (from 58 to 16 nmol Ca2+ per milligram protein per 30 minutes). Phosphorylase u activity remained elevated for 60 minutes after exposure to BrB (147 U/g protein). Evidence of elevated cytoplasmic Ca2+ is consistent with the inhibition of Ca'+-extruding/sequestering mechanisms at hepatocyte PM andlor ER. Prolonged elevation of cytosolic Ca2+ levels could overstimulate Caz+-sensitive processes within liver cells and thus initiate or contribute to hepatotoxic injury.