Glomerular permeability was investigated by using saccharated ferric oxide (Ferrivenin) as a tracer substance in sham operated and adrenalectomized rats during the infusion of normal saline and of renin and angiotensin. Kidney tissue was examined by electronmicroscopy. A striking increase in the number of the tracer substance particles was noted within the endothelial cells and basement membrane of sham operated rats infused with renin and angiotensin compared with that in adrenalectomized rats similarly treated or in rats infused with physiological saline only. No prominent change in the structure of the capillary wall was noted. More detailed information on the sieving characteristics of the glomerular filter and of the modifications induced by renin and angiotensin and by adrenalectomy was obtained by studying its behaviour in relation to the rates of passage of radioactively labelled PVP molecules of different sizes. In sham operated rats during renin or angiotensin infusion the passage of the higher molecular weight fractions of the polymer through the glomerular membrane was significantly increased, while the filtration of the lower molecular weight fractions was similar to that observed in rats infused with normal saline. One possible explanation could be an increase in intraglomerular pressure resulting in stretching of a small proportion of the total pore area. The absence of a similar response in adrenalectomized rats was attributed to the severe reduction in glomerular filtration rate and renal plasma flow caused in these animals by the infusion of renin and angiotensin.