Improved methods to measure 2H:1H and 18O:16O isotope ratios on microliter samples of biological fluids are described. Enriched levels of 2H (580%) and 18O (256%) in urine, plasma, saliva, and human milk can be measured with a precision of 3.2% (n = 200) and 0.97% (n = 200) and an accuracy of -4.6 +/- 4.4% (mean +/- SD, n = 200) and -0.32 +/- 0.87% (mean +/- SD, n = 200), respectively. Hydrogen gas samples are generated from 10 microL of undistilled fluid by zinc reduction in quartz reaction vessels. Water-CO2 equilibration of a 100-microL sample for 18O measurement is completed in 10 h using a modified commercial equilibration system. These methodological improvements facilitate and extend the use of 2H and 18O tracers in studies of body composition and energy expenditure.