Ibis paper presents the results of an anaIysis of dispersion in a porous medium, for the case when Dorcy's law is valid and the P&let number is large. It is shown tbat, after a sufficiently long time, the effective longitudinal diffusivity is a logarithmic function of the Pellet number. RBsume-Analyse de la dispersion d'un Bcoulement dans un milieu poreux, lorsque la loi de Dorey s'applique et pour des nombres de P&let considerables. On montre qu'apres un temps suffisant, la diffusivitd longitudinale est une fonction logarithmique du nombre de P&let. Zusammenfassun&--Untersuchungsergebnisse iiber die Durcbmischung in einem poriisen Medium werden mitgeteilt im Bereich des Dorcyschen Gesetzes und bei grosser Pecletzahl. Nach geniigend langer Zeit ist die effektive Liingsdurcbmischung eine logarithmische Funktion der Pecletzahl. 1. DESCRIPTION OF THE MODEL ONE of the difficulties in calculating the properties of flow through porous media is that the detailed geometry of the medium is not usually known and the flow field cannot be computed in detail. Theoretical work is at present confined to the investigation of models which can be handled mathematically. This note summarizes an investigation (SAFFMAN [z]) of the dispersion of a dynamically neutral material quantity in flow through a particular model, first considered by JOSSELIN DE JONG [l], and suggested independently by Sir GEOFFREY TAYLOR.