Copper toxicosis can occur in the absence of biliary copper excretion. To demonstrate whether biliary copper excretion capacity is correlated with hepatic mass and ATP7B function, we undertook studies in intact animals. Copper‐histidine was injected intrasplenically after baseline bile collection, followed by measurement of copper excretion in Long‐Evans Cinnamon (LEC) rats lacking atp7b function and in normal, syngeneic Long‐Evans Agouti (LEA) rats. The basal biliary copper excretion was very low in LEC rats compared with LEA rats, 8 ± 4 and 37 ± 18 ng copper/min, respectively; p < 0.05. After addition of copper, copper excretion increased significantly (by two‐ to five‐fold) in LEA rats during the 30 minute study period, whereas LEC rats showed only a slight and transient increase in copper excretion. After one‐third and two‐thirds partial hepatectomy immediately before copper loading, copper excretion decreased progressively. The studies indicate that biliary copper excretion depends on hepatocyte mass and ATP7B gene function. Analysis of copper excretion with our nonradioactive method will facilitate testing of novel therapies and pathophysiological mechanisms in copper toxicity. © 2000 John Wiley & Sons, Inc. J Biochem Toxicol 14:210–214, 2000