In the fed state, the net utilization of most amino acids by the ovine uteroplacental unit (defined as uterine uptake minus umbilical uptake) is zero. This study was to determine if this is true for the fasted pregnant ewe also, when uteroplacental glucose uptake is diminished. Glucose and amino acid concentrations were determined in 9 pregnant ewes across the uterine and umbilical circulations. Uterine and umbilical blood flows were determined by the steady state antipyrine diffusion method, and net uptakes across each circulation calculated by application of the Fick principle. Five amino acids were found to have a consistently greater uterine uptake than umbilical uptake. In addition, glutamate had a negative net umbilical uptake coupled with no net uterine uptake. Fasting resulted in an increase in the differences between the respective uptakes for all five amino acids. No additional amino acids were found to have disparate uptakes than were seen in the fed state. Umbilical glutamate uptake became less negative during fasting, but the difference between the uterine and umbilical circulations remained unchanged, as the uterine circulation took up glutamate during fasting. The umbilical circulation took up alpha-ketoisocaproate 0.626 +/- 0.18, fed), and the uptake was increased during fasting (1.8 +/- 0.6). These findings suggest that the branched chain amino acids and glutamine are utilized by the ovine uteroplacenta to a greater degree than are most other amino acids, and at increased rates during fasting of the ewe. These amino acids, through deamination, may be the source of uteroplacental ammonia production, as well as serving as an energy substrate when glucose supply is limited.