Stereological analysis of Leydig cell ultrastructure in aged humans was performed by a point-count method on testicular tissues fixed by perfusion with buffered glutaraldehyde. In six aged human males (74 yr old on the average), interstitial tissue occupied 38.9% of decapsulated testis volume, and Leydig cells constituted 3.1% to total tests volume. One cubic centimeter of the testis contained about 7.7 x 10(6) Leydig cells, each of which had a volume of 4,080 micrometer3 on average. Smooth endoplasmic reticulum of Leydig cells occupied 13.4% of cell volume and had a surface area of 2,474 cm2/cm3 testis tissue or 32,000 micrometer2/cell, which was 70.7% of the total membrane area of the cell. Occupying 5.8% of the cell volume, mitochondria had an inner membrane surface area of 434 cm2/cm3 tissue or 5,600 micrometer2/cell, which was 12.4% of the total membrane area of the cell. Relating these stereological values to published data on testosterone (T) secretion rate, an average human Leydig cell would secrete about 20 pg T/day. Each cm2 of smooth endoplasmic reticulum and mitochondrial inner membranes of human Leydig cells would produce 70 ng and 400 ng T/day, respectively.