The two main metabolites of amobarbital excreted in urine are 3′-hydroxyamobarbital (C-OH) and 1-(β-D-glucopyranosyl) amobarbital (N-glu). When testing the metabolite ratio in small single samples of urine, it was found that the urine in a Caucasian population contained about one-third glucose conjugation and two-thirds hydroxylation product, while an Oriental population excreted both metabolites in equal proportion. Attempts to learn the causes for the different metabolite ratios led to an investigation of metabolite concentrations in urine. The sums of the 2 metabolite concentrations were the same in both populations. The average urinary concentration of C-OH was greater in Caucasians than in Orientals, no matter how the data were expressed; the reverse was true for the N-glu metabolite. C-OH data was scattered more widely among Orientals than Caucasians; this might indicate bimodality of the distribution curves. There also was a trend toward more N-glu metabolite in urine of females than of males. Measuring the metabolite/creatinine ratios narrowed the distribution range of the data, particularly after correction for sex difference in creatinine, but population differences were not changed. Expected relationships between metabolite content of urine, sampling times, and plasma half-life (t½) were established by calculation. A Caucasian female with no capacity for N-glucosidation was found during the first part of this population survey.16 An Oriental male with only trace capacity for amobarbital hydroxylation was found in the second part.