The effects of high saline concentrations on the plasma osmolytes of the four populations of the Pacific Treefrog, Pseudacris regilla, were investigated. Two of the populations, Chris Wicht and Trabuco Canyon, were in fresh water environments and two populations, Newport Back Bay and Alkaline Marsh, were in brackish water environments. The Newport Backbay treefrogs were most tolerant of increased sodium chloride solutions with 11 of 15 individuals surviving at 240 mM sodium chloride at 72 hrs. The Trabuco Canyon population was the least tolerant with 10 of 15 specimens surviving 120 mM sodium chloride at 72 hrs but none at the 180 mM concentration at 72 hrs. The Chris Wicht and Alkaline Marsh treefrogs were intermediate in their survival times with frogs surviving in 180 mM saline solutions at 72 hrs. The Newport treefrogs appeared to be the most efficient in osmoregulating by mainstreaming the plasma sodium and increasing plasma urea to compensate for the osmotic gradient produced by the increased external saline concentrations. The Trabuco Canyon and the Chris Wicht treefrogs appeared to be intermediate in their osmoregulatory abilities with the Alkaline Marsh treefrogs being least efficient and more likely an osmoconformer. The difference in osmoregulatory responses appears to be a population difference and a function of habitat. Sodium chloride was the major plasma osmolyte in the four populations of treefrogs. Urea increased in importance as the external concentration of the saline media increased, thus the role of urea was significant in maintaining the internal osmoconcentration. These simple physiological mechanisms allow this widespread treefrog to occur in so many different habitats.