1. Body fat, fat-free mass and total body water of ten lactating women were estimated from deuterium-dilution spaces and from skinfold thickness measurements. Deuterium-dilution spaces were calculated from the 6 h (equilibration) and zero-time (extrapolation) deuterium enrichments in saliva, urine, human milk and breath water vapour samples. 2. The deuterium spaces obtained by equilibration were statistically larger than those obtained by extrapolation. Isotope dilution spaces derived from deuterium enrichments in saliva, breath water vapour and human milk did not differ with the exception of the 6 h equilibration value of milk, which was greater than that estimated from saliva. Deuterium-dilution spaces estimated from urine were consistently smaller than those derived from the other biological fluids. 3. No significant differences in body fat, fat-free mass and total body water were observed between anthropometric measurements and deuterium-dilution methods, except for extrapolated values derived from deuterium enrichments in urine.