Forty-five healthy men and women aged 16-39 and 59-89 yr were studied for total body water (TBW) and extracellular water (ECW); intracellular water (ICW) was calculated as the difference (ICW = TBW - ECW). An independent measurement of total body fat by inert gas uptake provided a value (+/- 2%) for fat-free mass (FFB is wt minus body fat). Results agreed with observations by others that TBW and ICW are lower in the aged and lower in women, whether expressed as absolute volumes or per unit of weight, surface area, or height. However, with FFB as the reference standard very different aging trends appeared. TBW/FFB remained constant to our oldest measured subjects (704 +/- 7 ml/kg). ICW/FFB was slightly lower at advanced age, but the 4-5% decrease for each sex was within statistical variability. With age ECW increased slightly and its proportion within the fat-free body (ECW/FFB) was significantly higher. Based upon FFB, the distribution, proportions, and aging trends of body water compartments were similar for men, women, and male rats. Although its potential limitations must be appreciated, the FFB appears widely useful as a reference standard. The stability of ICW volume and of fat-free mass in aging man does not support the hypothesis that cellular mass is lost by healthy mammals with age.