1. Normal diet and a simple short term prescribed diet were compared with respect to mean excretion of urine electrolytes and the variability in these responses in 12 healthy male subjects. Urine was collected from 7 to 15 h (overnight) and 15 to 25 h (daytime) after the start of dietary control.
2. Differences in mean responses were slight. For sodium and potassium excretion, however, there were tendencies for responses in subjects with high excretion on normal diet to decrease and for responses in those with low excretion on normal diet to increase towards the overall mean during the prescribed diet.
3. The prescribed diet reduced markedly the variability in electrolyte responses and for log transformed data reductions in variability were statistically significant in the daytime urine collections.
4. The results of this study have confirmed quantitatively the benefit of a short period of simple dietary control. The marked reduction in the variability of sodium and potassium excretion improves considerably the precision and sensitivity of studies of urine electrolyte composition.
