The neurotransmitter serotonin significantly contributes to the regulation of food intake and appetite behaviour. The cerebral serotonin synthesis depends on the availability of the precursor tryptophan. To examine how diets with different carbohydrate, protein and tryptophan content affect food preferences and the mood, healthy obese and non-obese male adults consumed the following isocaloric diets at breakfast: standard diet (60% carbohydrate), protein-rich diet (35% carbohydrate, 40% protein), carbohydrate-rich diet (80% carbohydrate), tryptophan supplemented diet (standard diet + 1.5 g tryptophan). The plasma concentration of tryptophan (TRP) and large neutral amino acids (LNAA) is determined by their uptake with the diet. In obese and non-obese adults the TRP/LNAA-quotient rose significantly by upto 0.40 to 0.57, following the tryptophan supplemented breakfast. The protein-rich diet reduced the TRP/LNAA-quotient. The preference of protein-rich food after the consumption of carbohydrate-rich diets (60-80% carbohydrate) was diagnosable with the non-obese person and not with the obese person. No obvious connection was detected between nutrient preferences and plasma TRP/LNAA-quotient. There was no influence of the plasma TRP/LNAA-quotient after meals with different nutrient relation on mood. Therefore it is concluded that different carbohydrate and protein content and also additional tryptophan supplementation of single meals is not able to modify the brain serotonin synthesis and release in healthy people in a kind that serotonin induced behaviour would be changed, at least on a short time basis.