The second harmonic generation activity of p-nitroaniline incorporated in zeolites strongly depends on the geometry of the zeolite internal pores, monodirectional zeolite being much more suited than bidirectional. Tridirectional zeolites are the worst host from the efficiency point of view. The relative efficiency for second harmonic generation increases almost one order of magnitude by incorporating p-nitroaniline into the channels of an oriented film of silicalite on a quartz substrate. (C) 2004 Published by Elsevier B.V.