If molecular transport of a lipophilic toxicant in isolated tissues is truly a diffusion process, then it should be possible to describe the process in terms of mathematical equations commonly used to describe the conduction of heat in homogenous and isotropic solids. The present approach compares well with other more direct and empirical methods for estimating diffusivity and it has the advantage that diffusivity so calculated is biologically realistic. The ability to evaluate toxicant uptake by organs of various sizes is theoretically factored in the method. Likewise, the physiological characteristics as a whole are factored by using the empirical data directly consistent with the stated theoretical framework.