In the framework of the COMAPS (COgnitive Management of anthropocentric Production Systems) project (Brite Euram BE 96-3941) this paper deals with an algorithm dedicated to the updating of rules bases under cognitive constraints. These constraints come from the assumption that the rules are reasonable approximations of those used by a human expert. In order to be able to take them into account, some special modification operations have to be designed. We present these operations as well as results obtained with data from one of the project pilot sites.