The effect of pregnancy on glomerular filtration rate and salt and water reabsorption in the rat.

1. Glomerular filtration rate (G.F.R.) and salt and water reabsorption were measured in age-matched (12- to 13-week-old) virgin rats and rats at different stages of pregnancy and infused with saline at 37.5 and 200 microliters/min. 2. G.F.R. and salt and water reabsorption were significantly higher in early pregnancy and remained at steady, high values until at least mid-pregnancy. G.F.R. was slightly lower close to term, but sodium and water reabsorption remained significantly enhanced. 3. Significant expansion of extracellular fluid volume (e.c.f.v) was first apparent during mid-pregnancy, indicating that e.c.f.v. expansion could not be directly responsible for the early increase in G.F.R. 4. Proximal tubules were significantly elongated 5- days after mating until term, indicating that an increased reabsorptive area in the proximal tubule may be at least partly responsible for the increased reabsorption in pregnancy.