A study was made on the hydration and serum electrolyte changes in 55 black underground gold miners who presented with heat disorders, and control data were obtained on 52 surface and 50 underground workers without symptoms. Cases were admitted for assessment and treatment, and a questionnaire was administered on symptoms, work, fluid intake, alcohol intake, recent health, and past history of heat disorders. Twenty-eight men had experienced heat disorders in the past. Blood specimens on days 0, 1, 2, and 7 for serum sodium, potassium, magnesium, calcium, inorganic phosphate, and serum total protein were obtained from 55 cases, of which 22 also had estimations of hemoglobin and hematocrit. Initial serum electrolyte levels, because of hemoconcentration, were found to be a poor indicator of underlying changes. Changes in serum total protein were used to correct electrolyte levels for dehydration, which showed deficits in serum total sodium and potassium. This method of correction, when compared with one using hemoglobin and hematocrit, showed similar but smaller changes in serum electrolytes. The cases were divided into subgroups of "cramps" and "collapse"; no significant differences were seen in ambient conditions, age, or electrolyte changes. The cramps group, however, had drunk significantly more water. The findings overall were those of dehydration and salt depletion.