Analyses of the chemical variations preserved in vertebrate teeth have been used to reconstruct a wide range of paleobiological and paleoenvironmental factors. Since teeth form by accretion, and are not remodeled during life, they record a time-series of the chemical variations experienced by an individual during tooth mineralization. Thus, chemical analyses of serial microsamples from teeth may potentially be used to reconstruct short-term (monthly to seasonal) changes in an individual’s biology, behavior, and/or the environmental condition it experienced. While researchers have examined a wide range of trace element and isotopic signals in teeth, one signal that has attracted particular interest is the oxygen isotopic ratio. The oxygen isotopic ratios of mammalian teeth have been shown to track the isotopic ratios of local precipitation, which in turn correlates with changes in mean annual temperature. Thus, the oxygen isotopic ratios of fossil teeth may provide information about paleoclimatic conditions. However, the precision of such environmental reconstructions is currently limited, in part, because the relative effect of environmental and metabolic processes on the oxygen isotope of many species have not been precisely quantified. We investigated a series of factors that could potentially influence the oxygen isotopic ratios of modern domestic horses. Since teeth are formed in equilibrium with body water, anything that influences the oxygen isotopes of body water would directly affect the isotope ratio of teeth. We thus examined the influence of several factors on the oxygen isotope of equine body water, including the oxygen isotope composition of drinking water and feed, as well as the influence of external temperatures and relative humidity. We have determined that the turnover rate of equine body water is approximately 10 days. While the oxygen isotope composition of drinking water clearly influences that of body water, we found that metabolic changes and external climatic conditions can alter an individual’s body water by ~2‰.