Since the introduction of methods based upon the use of stable isotopes, (Sprinson and Rittenburg 1949), protein turnover has been measured using many different protocols. Comparison of results obtained with different protocols show wide variability. A range of factors may influence the measurement of protein turnover although it is not clear the extent to which the variability might be attributed to differences in protocols or real differences in protein turnover itself. In this work attempts have been made to standardize the single and the intermittent dose method of protein turnover measurement and so assess the factors which contribute to its variation. The greatest source of vaiiation stems from the duration of measurement with differences of up to 50% being measured when an experiment of 9 hours is compared to one of 24 hours. Differences between long and short methods can in part be attributed to differences due to diurnal variation. During an intermittent dose method a step function has been identified in the enrichment of urinary ammonia which occurs between midnight and 6 am irrespective of the time of start of the study. Estimations of protein turnover made at the lower plateau will give values of protein turnover up to 50 % greater than protein turnover estimated from the higher second plateau. When comparisons were made in males and females, no differences were apparent in the rates of protein turnover calculated during a single and an intermittent dose method when the results were expressed /kg body weight. A positive correlation was however measured between flux, (units = [mgN/hour]), and body weight with body weight accounting for more than 70 % of the vanance in flux. Variation in protein turnover measured in a group of women was generally greater than that measured in a group of men. A factor in this vai iation was the cyclicity of the female hormones. Groups of women taking oral contraceptives experience different patterns of changes in protein turnover compared to women taking no oral contraceptives. A maximum value is achieved around the onset of menstruation in the contraceptive taking group, compared to the women who took no oral contraceptives who experience a minimum around this time. The stage of the menstrual cycle may account for 30% of the variation in flux. Many of the differences in protein turnover can be attributed to methodological differences. This work stresses the importance of careful standardization of methods and a consideration of the expression of results so as to clearly reflect the measurement which has been made.