>Any ideas ?

Yup, go to www.ibobjects.com and spend $30 on the Getting Started Guide -
it is really great on LookupCombos.

Geoff has already given you a detailed explanation, but here's some more.
Basically what you have to do is to have two TIB_Queries. Set KeyLinks of
your lookupdataset to MODELS.ID=STOCK.MODEL (or whatever you call your
fields) and KeySource to the datasource of your master query. Put a
TIB_LookupCombo into your application, set the required attributes and hope
for the best.

HTH,
Set