1. An assessment was made of the extent sodium (Na) and potassium (K) intake can be estimated from Na, K and creatinine (Cr) content of a second morning voiding urine (SMU) specimen collected within 4 h after the first voiding upon awakening but before breakfast in 159 clinically healthy, free-living individuals (20-79 years). The SMU and the rest of 24 h urine specimens for a 3-5 day period were collected. 2. The following equations for estimating 24 h urinary Na (24HUNaV) and K (24HUKV) excretions were developed, and the accuracy and the reliability of these equations were evaluated. Estimated value of 24HUNaV (mEq/day) = 16.3 square root of XNa; estimated value of 24HUKV (mEq/day) = 7.2 square root of XK, where XNa (or XK) = SMUNa (or SMUK)/SMUCr x predicted 24 h urinary Cr excretion. 3. Highly statistically significant correlations were detected between the values estimated and measured for both Na (r = 0.728, P < 0.001, n = 159) and K (r = 0.780, P < 0.001, n = 159). 4. These equations were applied to Group 1 subjects, who collected the urine for a single day, and to Group 2, for 3 days. The correlation coefficients between the values estimated and measured for Na and K were 0.531 and 0.443 in Group 1, and 0.821 and 0.590 in Group 2, respectively. No statistically significant differences were observed. 5. The SMU specimens provide a satisfactory alternative to both 24HUNaV and 24HUKV in adults for extensive epidemiological surveys but also for clinical application.