 the nature of empirical allometric expressions relating dispositional and kinetic parameters for a given xenobiotic across multiple mammalian species is well known . 
 it has also been demonstrated that a simple allometric relationship may be used to predict kinetic parameters for humans based merely on data for multiple xenobiotics from rats . 
 we decided to explore reasons for the variance in the data arising from the latter method . 
 we were particularly interested in learning whether any physicochemical characteristics of xenobiotics might account for outlying data points ( i.e. , poor prediction of human half - life from rat half - life ) . 
 we have explored the influence of lipid solubility as reflected by a xenobiotic 's log p value because adipose tissue comprises a significantly larger percentage of total body weight in humans than in rats . 
 we used half - life data from the literature for 127 xenobiotics . 
 a data subset of 102 xenobiotics for which we were able to find estimates of log p values , including several with extremely large log p values , was also analyzed . 
 first and second order models , including and excluding log p , were compared . 
 the simplest of these models can be recast as the familiar allometric relationship having the form y = a(xb ) . 
 the remaining models can be seen as extensions of this relationship . 
 our results suggest that incorporation of log p into the prediction of xenobiotic half - life in humans from rat half - life data is important only for xenobiotics with extremely large log p values such as dioxins and polychlorinated biphenyls . 
 moreover , a second order model in logarithm of rat half - life accommodates all data points very well , without specifically accounting for log p values.imagesfigure 1.figure 2.figure 3 . 
 afigure 3 . 
 bfigure 3 . 
 cfigure 3 . 
 dfigure 4 . 
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