1. Active electrolyte transport was examined in erythrocytes from women in the second and third trimesters of pregnancy and post partum, and compared with that in ovulating women.
2. There was a significant reduction in intracellular sodium ([Na]i) and increase in intracellular potassium ([K]i) in pregnancy with a return towards normal values in the post-partum period.
3. Maximum specific ouabain binding [number of Na+,K+-adenosine triphosphatase (Na+,K+-ATPase) units] was increased by 70% in pregnancy and returned slowly towards normal values post partum.
4. Na+,K+-ATPase activity as determined by ouabain-sensitive 86Rb influx in artificial media was also increased in pregnancy by 13%. It returned towards normal post partum.
5. The increases in Na+,K+-ATPase in pregnancy were not closely related to the concomitant increases in aldosterone or cholesterol nor to reticulocytosis and were not affected by 7 days of high (> 250 mmol/day) or low (< 50 mmol/day) sodium intake.