The maximal rate of activity of sodium extrusion by the sodium pump, Na+--K+ outward cotransport, passive permeability to sodium and potassium, Na+--Li+ countertransport, and passive permeability to lithium were measured in 45 essential hypertensive patients, 24 young normotensive subjects with at least one hypertensive parent and 24 normotensive subjects with both parents normotensive. 2. The maximal rate of activity of the sodium pump and the rate constants for passive permeability to sodium, potassium and lithium were similar in the three groups. 3. The mean value for Na+--K+ outward cotransport was significantly lower and that for Na+--Li+ countertransport significantly higher in the hypertensive patients than in the normotensive subjects without a family history of hypertension, but there was a great overlap between individual values. The offspring of hypertensive parents had intermediate values. 4. A highly significant positive correlation existed between Na+--K+ cotransport and Na+--Li+ countertransport both in the normotensive and in the hypertensive subjects, indicating that in the latter there were some with high Na+--Li+ countertransport and others with low Na+--K+ cotransport. 5. In nine hypertensive families studied, if an alteration of the transport system was detected in a hypertensive propositus, the same type of alteration was also found in his still normotensive offspring, thus indicating a familial tendency for the alteration to occur.
