Quantitation of the urinary metabolites of catecholamines, including VMA, HVA, and metanephrines, from six normal preschool children was performed during a normal diet, a restricted diet, and a diet with increased amounts of vanilla, vanillin, and phenolic acids. Ingestion of these substances has been suspected of producing elevated values of urinary catecholamines and their metabolites. Urine was collected on the fourth day of each diet in two consecutive 12-hour aliquots, beginning at 8:00 AM. Statistically significant diurnal variation in excretion of all three metabolites and total free catecholamines (epinephrine, norepinephrine, and dopamine) was demonstrated. Diet did not alter exceretion of total free catecholamines or any of the three metabolites. This study suggests that a three-day restricted diet is not necessary prior to screening children for neuroblastoma when using quantitative assay methods and that all screening tests should be performed on a 24-hour urine sample.