Renal haemodynamics and extracellular homeostasis during the menstrual cycle were studied in 14 healthy women (age 21-41 years) who were not taking oral contraceptives in the follicular (Period I) and luteal phase (Period II). The glomerular filtration rate [( 51Cr] EDTA clearance) and the effective renal plasma flow ([125I] hippuran clearance) increased from Period I to II by a median of 6.3% (95% confidence interval (CI): 0.6-9.2%) and 7.3% (95% CI: -0.4-22%) respectively. Serum sodium decreased from period I to II (p less than 0.01) by a median of 1 mmol/l (95% CI: -2.0 to -0.5 mmol/l) and the urinary excretion rate of potassium increased (p less than 0.02) from a median value of 35 mumol/min in Period I to 45 mumol/min in Period II. The extracellular fluid volume did not change between the two periods but the concentration of water in serum increased (p less than 0.05) from a median value of 91.7-92.0 g/100 g in Period II. Serum total protein and serum albumin both showed a borderline statistically significant decrease from Period I to II. The investigation demonstrated a number of physiological and biochemical changes from the follicular to the luteal phase, most of which in a lower scale mimic well known changes that occur during pregnancy.