The balance of vascular tone of the afferent and efferent arteriole is a crucial determinant of glomerular hemodynamics. Despite the intimate anatomical relationship of the two arterioles in the JGA, the mechanisms that regulate afferent and efferent arteriolar tone are different. In the afferent arteriole, two intrinsic mechanisms, the myogenic response and macula densa-mediated TGF, play a dominant role, maintaining the GFR at a constant level over a wide range of renal perfusion pressure. Studies have shown that these two mechanisms are modulated by NO. In addition, an interaction between TGF and Ang II seems to be essential in maintaining GFR constant despite large variations in daily intake of salt and water. In the efferent arteriole, Ang II is one major factor involved in the control of vascular resistance. In addition, recent studies have provided evidence that NO and PGs produced by the glomerulus may control resistance of the downstream efferent arteriole. Since the early segment of the efferent arteriole resides within the glomerulus, various autacoid hormones produced by the glomerulus may reach and directly act on this segment, thereby controlling the glomerular capillary pressure. Thus, it would be important to understand the differences in the mechanisms operating at the afferent and efferent arteriole, as well as their alterations in various physiological and pathological conditions.