SUMMARY1. Daily changes in water and electrolyte balance during pregnancy were investigated in rats housed in metabolism cages.2. Fluid intake was significantly elevated above control values from day 13 of pregnancy, with urine output failing to be raised to the same extent. This would result in an extensive fluid retention if extrarenal fluid losses were not substantially altered.3. Electrolyte intake increased from as early as the third day after mating with an accompanying increase in renal ionic excretion. A net retention of Na, Cl and K did not occur until the final week of pregnancy when the urinary output of these ions was reduced.4. In a parallel study, changes in plasma volume and composition throughout pregnancy were investigated. 5. A significant increase in plasma volume occurred from day 6 of pregnancy at a time well before fluid intake or urine output were altered. This indicates either an altered extrarenal output or. a shift of fluid between body fluid compartments.6. Maternal plasma sodium and total osmolality were reduced during the last week of pregnancy despite the salt retention, suggesting an increased fetal usage.7. Such findings are related to the known renal and endocrine changes of rat pregnancy.