The metabolic ratios (MRs) between debrisoquine (DBQ) and 4-hydroxydebrisoquine in urine after a single dose of 10 mg DBQ was determined in 175 unmedicated, healthy subjects older than age 59 (mean of 75 years). Creatinine clearance was determined on the same 8-hour urine collection. Test procedures were well tolerated in all cases. Although age was significantly correlated with creatinine clearance (r = -.38), there was no relationship between age and MR. Analysis by kernel density estimation revealed a bimodal distribution of MRs with an antimode of 11.6. Six subjects (3.4%) were categorically slow DBQ metabolizers (MR greater than 11.6). The proportion of elderly slow metabolizers approaches the lower range determined in a younger population. Our findings, that DBQ oxidative metabolism does not necessarily change with aging, alone, and that (genetic) slow DBQ metabolizers endure into old age, remaining at risk for treatment with many commonly used psychotropics, suggests the need to study the relevance of metabolic phenotyping in elderly psychiatric patients.