When internal contamination with a radionuclide is investigated by the analysis of urine, it is essential to know the period of metabolic activity represented by each sample of urine. A genuine 24-hr sample is the ideal, but it is not always possible to be certain that complete, accurately-timed 24-hr collections of urine have been made. The best internal standard appears to be the normal creatinine content of urine. A comprehensive consideration of the literature on human creatinine excretion is presented. The creatinine excretion rate of an individual person is almost constant and is correlated with the potassium content of the body. When the excretion of an individual has not been measured, standard values for creatinine excretion rate of 1.7 g/day for men and 1.0 g/day for women may be used but from these values, the sample time is only reliable to within a factor of 0.5–1.5 (95 per cent limits of confidence).