Urinary excretion of various catecholamine metabolites (4-hydroxy-3-methoxymandelic acid, homovanillic acid, 3-methoxy-4-hydroxyphenylglycol) was studied in preterm infants with idiopathic apnea treated with theophylline. Relief of apnea in these infants was not associated with significant increase in the urinary excretion of various metabolites. We suggest that theophylline does not seem to relieve apnea of prematurity by the stimulation of peripheral catecholamine systems.