The apparent absorption of copper was determined in seven elderly men using the stable isotope 65Cu. Analysis of isotopic ratios of copper was done using thermal ionization, magnetic sector mass spectrometry after separation of copper from fecal samples by ion exchange chromatography. Mean apparent copper absorption during the first metabolic period in five men was 23.9 +/- 1.4% (mean +/- SEM) when fed a semipurified diet containing 3.3 mg of copper daily and 9 to 19 g nitrogen from egg albumin. Mean apparent copper absorption in five men during the second metabolic period was slightly but significantly higher, 27.7 +/- 0.31% (mean +/- SEM) when their diet contained 3.3 mg of copper per day and 4.0 to 5.7 g of nitrogen from egg albumin. The quantity of copper absorbed was equivalent to 0.8 mg per day in the first metabolic period and 0.9 mg during second metabolic period and was sufficient to maintain copper balance in these elderly men.