To determine whether the caloric source of intravenous nutrition can influence oxidative drug metabolizing capacity, antipyrine metabolism was studied in six healthy volunteers, who were taking no food or liquid by mouth, after they had been administered an intravenous nutritional regimen of 5% dextrose, 440 kcal/day, for 4 days and after they had been switched to an essentially isocaloric intravenous nutritional regimen of amino acids (Aminosyn® 3.5%) for 1 day. The change in intravenous nutritional regimen resulted in a 21% decrease in mean half-life (range: 3–32%), a 20% decrease in mean area under the concentration-time curve (range: 4–42%), and a 24% increase in mean metabolic clearance rate (range: 2–71%) for antipyrine. These results show that the change from intravenous dextrose to intravenous amino acids for only 1 day produced in all subjects an increase in antipyrine metabolism. Interestingly, there was marked variability in the responsiveness of the different subjects to the change in intravenous caloric source.