Teflon digestion bombs have been used for pressure decomposition of rat liver samples preliminary to copper analyses by atomic absorption spectrometry. The analytical procedure is convenient and is consistently free from copper contamination. The recovery of copper added to liver samples (three mug Cu added per g wet wt) averaged 102 (S.D. plus or minus 3) percent. The accuracy of the copper analyses was verified by use of National Bureau of Standards reference bovine liver. The mean concentration of copper in perfused livers of 16 untreated male rats averaged 16.1 (S.D. plus or minus 2.3) mug per g dry wt. The mean concentration of copper in perfused livers of 8 male rats which received estradiol-17 beta in dosage of 50 mug per day, s.c., for 21 consecutive days was 21.7 (S.D. plus or minus 4.7) mug per g dry wt (p vs controls equals greater than 0.01). This study demonstrates that estrogen administration can cause a significant increase in the concentration of hepatic copper.