The effect of dietary threonine intake on phenylalanine oxidation, measured during a primed, constant infusion of L- (1-\sp?C) phenylalanine, was determined in 25 Yorkshire piglets receiving total parenteral nutrition. Phenylalanine oxidation by rate of \sp?CO\sb2 released in breath decreased linearly as threonine intake increased from 0.05 to 0.2 g/kg/d with no change thereafter. A safe level of threonine intake was estimated as 0.212 g/kg/d by the upper 95% confidence interval of the break point analysis. These data suggest that (1) threonine requirement of neonatal piglets during TPN is less than 70% of oral requirement (2) indicator amino acid oxidation could be used as a new technique to determine amino acid requirements of human infants receiving total parenteral nutrition. The preliminary threonine kinetic study included 10 piglets. Threonine pool size was estimated as 4.3 mmol and turnover time as 3.6h. A primed constant infusion rate for threonine was estimated as 20 and 4 μCi/kg/h, respectively, and for glycine as 7 and 7 μCi/kg/h respectively.