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The stability of creatinine excretion over periods of 4, 8 and 24 hr was studied in 12 subjects, including the 2 investigators; the literature on the subject was critically reviewed. The evidence accumulated shows that creatinine excretion is much less stable than is often assumed. Some subjects are stable excretors as assessed by sequential 24-hr excretion rates, but very few are stable as assessed by sequential rates over periods of less than 24 hr. Not all within-day variability is random. Significant or highly significant diurnal variations were found in 7 of 12 subjects in the present study, in several other recent reports, and consistently in raw data published early in the century.For decades there was little doubt about the constancy of creatinine excretion. Urine samples were unhesitatingly discarded as invalid if the creatinine values exceeded some arbitrary range, or the creatinine