The controlsource you mention above is for the grid, right?

Yes - for the grid column with the combo in it - that way you can leave its Sparse property set to .F., but still display the descriptive text from the lookup table instead of the foreign key field in the grid's RecordSource when the cell does not have focus.

Where do you update the recordsource table? In the combobox interactivechange?

You do not need to write any code to do this - you get it for free. I am assuming that the field in the grid that you are updating is a foreign key reference to some other ( lookup ) table. All you need to do is set the combo's ControlSource to the foerign key field in the grid's RecordSource.