Direct measurements of the pressures and flows governing the formation of glomerular ultrafiltrate have been made possible in recent years by virtue of 1) the discovery of rats and monkeys possessing glomerular capillaries on the renal cortical surface, accessible to micropuncture, and 2) technological advances that permit measurement of intracapillary hydraulic pressure and assessment of the change in colloid osmotic pressure along the glomerular capillary network. Based on these direct measurements, evidence has been obtained to indicate that glomerular capillary hydraulic pressure and hence the net driving force for ultrafiltration are lower than previously believed. By the efferent end of the glomerular capillary network, net filtration of fluid ceases, owing to a reduction in the net driving force to zero. Evidence in the rat indicates that the process of ultrafiltration is highly dependent on glomerular plasma flow rate. Studies in rats with surface glomeruli have also made possible an assessment of the factors that govern the transport of macromolecules across the highly specialized capillary network. In addition to molecular size, transcapillary movement of macromolecules is influenced by the glomerular filtration rate, since total transport reflects the combined contributions of convection as well as diffusion. Molecular charge has also been found to be an important determinant of the transport of macromolecules, very likely contributing to the marked restriction to the transcapillary movement of albumin. This electrostatic restriction to the transport of polyanions such as albumin, by some fixed, negatively charged component(s) of the glomerular capillary wall, is markedly reduced in primary glomerular injury. Evidence indicates that glomerular injury results in loss of these fixed negative charges from the capillary walls, providing an attractive explanation for the enhanced filtration of albumin, and hence the proteinuria, observed in a variety of glomerulopathic states.