Glomerular filtration rate (g.f.r.), renal plasma flow (r.p.f.), salt and water reabsorption and plasma concentrations of progresterone and prolactin were measured in virgins, 19 day pregnant and lactating rats. 2. G.f.r. was raised by about 40% in animals at 6-7 days of lactation when compared with virgins but towards the end of lactation returned to virgin levels. Salt and water reabsorption followed a similar pattern. 3. At the time of maximum change of g.f.r. during lactation r.p.f. was raised by about 30% and so filtration fraction rose by about 10% when compared with virgins. 4. Plasma prolactin and progesterone concentrations rose early in lactation but remained elevated when the renal changes had returned to normal. 5. Factors other than changes in prolactin and progesterone concentrations must be implicated in the changes in renal function that occur during lactation.
