If the field is empty, the value IS null and this is the default value of fields in a new record unless an auto-enter field option enters a different value.
Or do you want to enter the text "NULL"?
If so, open manage | Database | Fields and double click each field to open field options. Select the auto-enter tab and enter NULL (no quotes) into the Data box.
Yes, it was the text I wanted, Phil. I knew about "auto-enter" but I thought that might replace any existing text. I guess auto-entered text only gets applied to empty fields.
Thanks for your help!
Auto-entered text from the Data box is only entered into new records. Anything specified there will have no effect on existing records.
And for some needs, you might choose to use layout text placed on top of the field with conditional formatting to make the text disappear when the underlying field is not empty. This, for example, would allow you to display "NULL" for an empty date field.