The rat renal artery and abdominal aorta have been studied by light and electron microscopy. In rats of 200 g body weight the extracellular space in aortic media ranges between 50-60% and that of the distal renal artery between 15-25%. The surface to volume ratio of aortic smooth muscle cells is 2.7 micron2/micron3 compared to 1.6 micron2/micron3 in the distal renal artery. Dense bodies are rare in aortic smooth muscle cells but are abundant in those of the distal renal artery. Other ultrastructural details of the smooth muscle cells are similar in the two types of artery. Cell-to-cell contacts consist of simple apposition of plasm membranes and their number is proportional to the total length of cell membrane profile. Mitochondria represent 7-8% of the cell volume in both arteries. The proximal renal artery shows structural characteristics which are intermediate between those of the aorta and distal renal artery. In all renal arteries examined, bands of longitudinal smooth muscle are present in the adventitia, principally at branch points. In older rats, regions of discontinuity of the internal elastic lamina have been observed.