We evaluated the glomerular filtration rate (GFR) during the second postpartum week in 22 healthy women who had completed an uncomplicated pregnancy. We used physiologic techniques to measure GFR, renal plasma flow and oncotic pressure, and computed a value for the 2-kidney ultrafiltration coefficient (K_f). We compared these findings to pregnant women previously studied on the first postpartum day as well as non-gravid women of reproductive age. Healthy female transplant donors of reproductive age permitted the morphometric analysis of glomeruli and computation of the single nephron ultrafiltration coefficient (SNK_f). The aforementioned physiologic and morphometric measurements were utilized to estimate transcapillary hydraulic pressure (P) from a mathematical model of glomerular ultrafiltration. We conclude that postpartum day 1 is associated with marked glomerular hyperfiltration (+41%). A theoretical analysis of GFR determinants suggests that depression of glomerular capillary oncotic pressure, the force opposing the formation of filtrate, is the predominant determinant of early elevation of postpartum GFR. A reversal of the gestational hypervolemia and hemodilution, still evident on postpartum day 1, eventuates by postpartum week 2. An elevation of _GC to supernormal levels ensues, yet GFR remains modestly elevated (+20%) above non-gravid levels. An analysis of filtration dynamics at this time suggests that either a significant increase in P by up to 16%, or an ~50% increase in K_f, or a combination of smaller increments in both, must be invoked to account for the persistent hyperfiltration.