Ultrastructural features of juxtaglomerular cells have been correlated with plasma and kidney analyses from non-hibernating, hibernating and awakening ground squirrels. Juxtaglomerular cells in kidneys from hibernating animals show signs of increased activity. Plasma samples from hibernating animals show a significant increase in magnesium. Kidney analyses from hibernating animals, show glycogen increases and lactate and inorganic phosphate decrease significantly. Adenosine triphosphate remains the same. Maintenance of high-energy phosphates in the hibernating kidney is essential to maintaining sodium transport and osmotic pressure. This coupled with a functional renin-angiotensin system regulates water and electrolyte balance.