[Dependence of the serum concentrations of creatinine and urea on the time of day, with normal and impaired kidney function (author's transl)].

Daily variations in the serum concentrations of creatinine, urea and cortisol were studied in 7 patients without kidney or muscle disease, and in 8 patients with impaired kidney function (creatinine clearance less than 40 ml/min). Neither creatinine nor urea showed a circadian rhythm; the daily variations lay within the limits of the methodological scatter. Serum cortisol showed its known circadian rhythm in high amplitude.