The mesangial uptake and transport of particulate material has been studied using an iron-dextran complex, Imposil, as a tracer particle. The tracer was given as a single dose into the tail vein of the mouse, and animals were killed at intervals from 5 minutes to 48 hours. Light and electron microscopy showed that the iron-dextran complex was initially taken into the matrix channels of the mesangium from which it progressed over the course of 8 hours to the matrix of the juxtaglomerular apparatus and intercellular spaces of the macula densa. This delineated a continuous functional pathway from the glomerular capillary lumen to the macula densa cells of the distal tubule for material taken up by the mesangium. It is suggested that the products of inflammatory lesions in the glomerulus could affect the secretory function of the granular cells of the juxtaglomerular apparatus as it would appear that such products must circulate in the immediate environment of these cells.