BACKGROUND The feeding in the first months of the life seems to influence the risks of obesity and affinity to some diseases including atherosclerosis. The mechanisms of these relations are unknown, however, the modification of hormonal action can likely be taken into account. Therefore, in this study the levels of ghrelin and orexin A - peripheral and central peptide from the orexigenic gut-brain axis were determined. METHODS Fasting and one hour after the meal plasma concentrations of ghrelin and orexin were measured in breast-fed (group I; n = 17), milk formula-fed (group II; n = 16) and highly hydrolyzed, hypoallergic formula-fed (group III; n = 14) groups, age matched infants (mean 4 months) as well as in children with iv provision of nutrients (glucose - group IV; n = 15; total parenteral nutrition - group V; n = 14). Peptides were determined using EIA commercial kits. RESULTS Despite the similar caloric intake in orally fed children the fasting ghrelin and orexin levels were significantly lower in the breast-fed children (0.37 +/- 0.17 and 1.24 +/- 0.29 ng/ml, respectively) than in the remaining groups (0.5 +/- 0.27 and 1.64 +/- 0.52 ng/ml, respectively in group II and 0.77 +/- 0.27 and 2.04 +/- 1.1 ng/ml, respectively, in group III). The postprandial concentrations of ghrelin increased to 0.87 +/- 0.29 ng/ml, p < 0.002 and 0.76 +/- 0.26 ng/ml, p < 0.01 in groups I and II, respectively as compared to fasting values. The decrease in concentration of ghrelin after the meal was observed only in group III (0.47 +/- 0.24 ng/ml). The feeding did not influence the orexin concentration. In groups IV and V the ghrelin and orexin levels resembled those in milk formula-fed children. CONCLUSIONS The highly hydrolyzed diet strongly affects fasting and postprandial ghrelin and orexin plasma concentrations with possible negative effect on short- and long-time effects on development. Also total parenteral nutrition with the continuous stimulation and lack of fasting/postprandial modulation might be responsible for disturbed development in children fed this way.