Glomerular ultrafiltration of the plasma is a fundamental component of vertebrate renal function. The importance of the glomerulus is reflected by its near-universal presence and great elaboration among the vertebrates. Although the general structural features and functional properties of the glomerulus appear to be largely similar among diverse groups, there exists considerable variation in the magnitude of the rate of filtration. The kidney is the primary vertebrate organ responsible for water and metabolic waste excretion, and glomerular filtration plays an important role in these functions. Therefore, the magnitude of the GFR appears to be influenced primarily by the rates of water influx and metabolism. Major phylogenetic differences in morphological, physiological and metabolic design have a decisive impact on the magnitude of the GFR. The endothermic classes, with more numerous glomeruli, high metabolic rates, and high ultrafiltration pressures, have proportionately higher rates of glomerular filtration than the ectothermic groups. As a group, the reptiles, with presumably the lowest rates of water influx, exhibit the lowest GFRs. Within each class, there are trends toward species with greater access to free water having higher GFRs (e.g. fresh water vs. marine; mesic vs. xeric. The clearest examples exist for the teleosts, with marine forms having lower GFRs than their fresh water relatives. The coupling of the GFR to environmental influences is also demonstrated by the response of the animal to environmentally imposed perturbations, such as dehydration. In terrestrial animals during dehydration, reductions in the rate of glomerular filtration occur reducing the rate of urinary water loss. And increases in GFR appears to be important in the rapid elimination of water loads in nonmammalian vertebrates. This short-term modulation of the GFR occurs by either changing glomerular plasma flow or glomerular capillary hydrostatic pressure, or both. In addition, shifts in the filtering populations of glomeruli can take place, as has been demonstrated in birds. Although the mediators of these effects have not been unequivocally identified, several hormones, including antidiuretic hormone, angio-tensin, and catecholamines, have been implicated.