>>I have a logical field, which I want to populate with .t. on some condition and leave blank otherwise.
>
>Nadya,
>a blank logical field evaluates the same as a .F. Plus a logical field is only a 2 state thing, either true or false.... not 3 state, true, false, or blank.

This is not so. Every logical field actually has three states, .T., .F. or BLANK. Blank can be test with ISBLANK() and can be set with the BLANK command (and is the default status for a newly appended record). If the logical field is NULL enabled then it can have four states, the three above as well as .NULL. which can be set with REPLACE ... WITH .NULL. and can be tested with ISNULL(). They are all mutually exclusive states.