The aim of this study was to investigate absorption and retention of nitrogen, fat, calcium, phosphorus and magnesium, as well as plasma amino acid concentrations in 19 preterm infants fed three experimental protein hydrolysate preterm formulas (PTHF): seven received a preterm formula based on 100% whey hydrolysate protein (PTHF1), seven other infants were fed a preterm formula based on a mixture of 78% whey and 22% casein hydrolysed protein (PTHF2) and a third group of five infants were fed the same type of protein hydrolysate (78/22) enriched with histidine (PTHF3). Metabolic balances (n = 39) and plasma amino acid concentrations (n = 12) in preterm infants fed a standard preterm formula (whey/casein: 60/40) were included as a control group. Amino acid composition of the formulas was determined after complete hydrolysis with 12 N HCl. Compared with the standard preterm formula, the use of protein hydrolysate formulas led to a decrease in nitrogen and phosphorus absorption without modification of retention. Net absorption of calcium and magnesium was not significantly different in the four groups but calcium intake necessary to obtain calcium retention similar to the standard preterm formula was higher in the infants fed the hydrolysate formulas. Plasma amino acid concentrations were related to the amino acid composition of the formulas. Compared with the standard preterm formula, all three protein hydrolysate formulas led to a significant increase in plasma threonine, and a decrease in tyrosine and phenylalanine concentrations. In addition, there was an important reduction in plasma histidine concentrations with PTHF1.(ABSTRACT TRUNCATED AT 250 WORDS)