A quantitative relation between the oxygen isotope content of environmental water and the oxygen isotope content of body water was sought to determine the oxygen isotopes in the phosphate of mammalian bones and teeth as a tool in paleoclimatic studies. A correlation was found between the ₁₈O content of rat blood water and that of the drinking water. A steady-state condition was attained in the blood approximately one week after altering the isotope content in the drinking water. A correlation between the blood and drinking water₁₈O concentrations in persons living in different areas was also observed. The suggested relation between the ₁₈O of body water and environmental water apparently is real and is probably characteristic of each species of mammal. The P₁₈O₄_3- values measured in human bones from individuals in different geographical latitudes varied over a wide range, inconelation with the environmental₁₈O in the water. All the results substantiate the initial hypothesis. -- AATA