The problem of the waiting time effect in the relaxation under field of the zero-field-cooled magnetisation of ultrafine particle systems is investigated using a Preisach-Neel-type model. In the Preisach plane, one uses a double-well potential distribution calculated with the Stoner-Wohlfarth model. Static and dynamic interactions for ultrafine particulate systems are taken into account. The simulated results are in good qualitative agreement with the experimental ones which attest that the ageing effect is related to the existence of the interactions between particles.