This study set out to examine the diurnal pattern of electrolyte excretion in urine, and to investigate whether urine collected from after dinner until next morning (half-day evening urine) might to be an alternative to 24-h urine more valid than overnight urine for ranking a person's electrolyte status. In a cross-sectional study of 40 young adults, two consecutive 24-h urine samples were collected, followed one month later by two further consecutive 24-h samples. Mean excretion rates of sodium and chloride steadily increased during the daytime. Electrolyte/creatinine ratios also increased, but all excretion rates decreased after sleep. The potassium excretion rate increased rapidly in the morning but declined steadily after 10 a.m.. Estimates of 24-h electrolyte content calculated from half-day amounts were approximately 80-92% of the actual 24-h urinary electrolyte values. Estimates obtained using overnight urine amounts approximated 60% of the actual 24-h values. Correlation coefficients between actual 24-h urinary electrolytes and estimates based on half-day urine were better than correlates between actual 24-h amounts and estimates obtained using timed overnight urine. These results suggest that half-day evening urine may be a useful alternative to 24-h urine for assessing electrolyte in-take.