1. Studies to explain a significant socioeconomic and rural-urban blood pressure gradient found in central Mississippi are reported.
2. 24 h sodium excretion alone fails to explain the differences found, and fails to correlate with blood pressure.
3. A single 24 h determination of electrolyte excretion correlates modestly with the mean of 6 days' collection. Three days' collection and blood pressure determination is a reasonable compromise.
4. The urinary Na/Ca ratio is significantly higher in specimens from rural black girls compared with urban girls, and significantly higher in specimens from those with systolic pressure > 125 mmHg compared with those with < 105 mmHg pressure.
5. The urinary Na/K ratio of black girls was significantly higher than that of white girls of the same age, as was their blood pressure.
6. No correlation between blood pressure and urinary electrolytes could be found in black women aged between 35 and 45.