Background: Feeding extensively hydrolysed infant formula influenced the availability of some amino acids (AA) in term infants(Decsi et al., J Pediatr Gastroenterol Nutr 22: 62-67). It is not known whether PHF has similar effect. Subjects: Term infants fed conventional formula (F, n=11) or PHF (n=11).Intervention: Investigations at the ages of 5, 30, 60, 90 and 120 days. AA were measured with HPLC. Results: Gains in weight, length and head circumference and plasma total protein, urea nitrogen and creatinine concentrations did not differ. At day 120, infants fed PHF showed significantly lower plasma albumin concentrations than infant receiving F (43.3±3.4 versus 48.9±3.5, g/l, mean±SD, P<0.05). Plasma concentrations of lysine [day 90: 117 (21) versus 143(24)], threonine [day 30: 140 (53) versus 263 (87)] and tyrosine [day 30: 52(17) versus 77 (20), day 60: 56 (14) versus 87 (19), day 90: 46 (9) versus 81(17)] were significantly lower in infants receiving PHF than in those fed F[μmol/l, median (IQR), P < 0.05]. Conclusion: Feeding PHF did not affect growth, some parameters of protein metabolism and the majority of AA. However, plasma concentrations of lysine, threonine, tyrosine and albumin were at least at one occasion lower in infants fed PHF than in infants fed F.