The aim of this work is to describe quantitatively, from a physical point of view, molecular, molecular transport in thyroid slices. The study of the release of [3H]sucrose, [3H]inulin and 131I-labeled albumin leads to the following conclusions: the molecular transport in the slices is not due mainly to free diffusion. Indeed as these molecules are retained in the interfollicular spaces, the transport of matter is a mechanical process due to the agitation of the medium in which the slices are incubated. It depends on the elastic properties of the thyroid tissue, the frequency of the agitator and the thickness of the slice. This transport process can be described by a diffusion equation with an empirical diffusion coefficient; we call it diffusivity. These findings must be taken into account in any in vitro kinetic study of thyroid metabolism and of its regulation by effectors such as thyrotropin. The possibility of interference of such mechanical processes in the interpretation of kinetic tracer studies with tissue slices or fragments should be considered.