Studies on glomerular filtration in the Munich-Wistar rat, a strain with surface glomeruli, have permitted for the first time the evaluation of the four factors that determine filtration rate: 1) the rate of nephron plasma flow (RPF), 2) the glomerular hydrostatic pressure gradient (deltaP) from capillary to Bowman's space, 3) systemic oncotic pressure (piA), and 4) the glomerular permeability coefficient (LpA), and their respective roles in producing change in filtration rate. The process of filtration in the Munich-Wistar rat is characterized by filtration pressure equilibrium and, therefore, filtration should be highly plasma flow-dependent. Analysis of data from this strain suggests that multiple factors can change LpA, and, at least in animals which do not achieve filtration pressure equilibrium, these alterations in LpA can influence the final filtration rate. Multivariate statistical analysis in a variety of experimental conditions in the Munich-Wistar rat suggest that changes in RPF remain the major mediator of change in nephron filtration rate, both in rats at filtration pressure equilibrium and in disequilibrated animals. Although the glomerulus is an ultrafilter driven by hydrostatic forces, changes in deltaP assume a lesser role in producing alterations in filtration rate.