AbstractKidney samples were obtained from ten hibernating and ten control 13‐lined ground squirrels.Citellus tridecemlineatus. The tissue was fixed in cold buffered 2% osmium tetroxide, dehydrated in graded alcohols and embedded in Maraglass. Sections were made with glass knives on a Porter‐Blum microtome and viewed with RCA‐EML and RCA‐EMU 3 electron microscopes. Observations and comparisons were made of the glomerular capillaries in samples from both groups. In comparison with data on other animals reported in the literature, the basement membrane of the renal glomerulus of the control ground squirrel is approximately twice as thick, averaging 2,000–2,500 Å in width. Hibernation results in several structural modifications of the glomerular ultrafilter as follows: (1) the endothelial pores are decreased in number and size; (2) the podocytic foot processes are swollen and the slit pores reduced in size; and (3) the basement membrane thickens, averaging 4,000–5,000 Å in width, and becomes irregular in contour. It is concluded that the porosity of the glomerular ultrafilter decreases during hibernation. This aids in explaining the decrease in the rate of urine formation which occurs during this time of lowered body temperature and metabol