Plasma ratios of tryptophan (Trp) and tyrosine (Tyr) to their respective competing large neutral amino acids (LNAA) for brain uptake, serum insulin and plasma glucose concentrations were determined in 31 fasting healthy female subjects, and in two smaller groups of smokers and oral contraceptive users, who were subsequently allowed to compose individual breakfast meals from a selection of 25 dietary products. Additional blood samples were collected at 2 hr after the meal. Smokers consumed less carbohydrate (-22%) and total calories (-23%) and showed decreased basal serum insulin level, when compared to controls on the same age. Females on oral contraceptives consumed significantly more carbohydrate (+54%) and total calories (+32%) than comparable controls. In the 31 females there was no significant correlation between any of the biological variables and the intake of fat or total calories. The ratio of carbohydrate/protein eaten was significantly and directly correlated with age and with the sum of plasma ratios Trp/LNAA and Tyr/LNAA, and these independent variables associated with 37% of the variance in the ratio carbohydrate/protein consumed, as evaluated by multiple regression analysis. After the meal, the plasma ratio Tyr/LNAA was increased, whereas the ratio Trp/LNAA was decreased in subjects whose ratio carbohydrate/protein consumed was below the mean of the full sample, whereas subjects who consumed meals with a high ratio carbohydrate/protein showed an increase in plasma ratio Trp/LNAA. It is concluded that biological variables in man are significantly associated with the choice between nutrients with different carbohydrate and protein contents for breakfast. The changes in the plasma ratios Trp/LNAA and Tyr/LNAA after consumption were generally moderate.