The capacity for debrisoquin metabolism was determined in 377 healthy Spanish volunteers by measuring the amount of debrisoquin and its main metabolite, 4-hydroxydebrisoquin, in urine after an oral dose of debrisoquin. Debrisoquin oxidation was polymorphic, with 25 subjects (6.6%) phenotyped as poor metabolizers whereas 352 subjects (93.4%) were classified as extensive metabolizers. The metabolic ratio between debrisoquin and 4-hydroxydebrisoquin (percent of dose) in 6-hour urine samples ranged from 0.03 in extensive metabolizers to 93.5 in poor metabolizers. The proportion of poor metabolizers found is in the range observed in other white populations studied.