Microwave effects on free amino acid concentrations in milk versus a water bath heating were investigated in view of their importance for infant growth. Concentrations of few amino acids, such as aspartate, serine or lysine, are unchanged whatever the way and the temperature of heating. In contrast, tryptophan concentrations decreased similarly whatever the way of heating (110 +/- 3 mumol/l before heating vs 84 +/- 4 mumol/l after 30 degrees C microwave heating, p < 0.05). On the contrary, concentrations of glutamate and glycine increased more after water bath heating at 90 degrees C (325 +/- 4 and 101 +/- 1 mumol/1, respectively) than after microwave heating (312 +/- 4 and 95 +/- 1 mumol/l, respectively, p < 0.05) suggesting milk proteolysis. Moreover, the accumulation of ammonia observed at 90 degrees C with the water bath together with increase Glu levels might reflect a degradation of glutamine. An ornithine enrichment, more evident with microwave heating, was shown and could be of interest as it is a polyamine precursor. Also, considering few variations of free amino acid concentrations and the time saved, microwave heating appears to be an appropriate method to heat milk.