Rm....

In the textbox valid event....

if myField = 'Y' or myField = 'N'
return 1
else
messagebox('Must be Y or N',48,'Oops')
return 0
endif

If you have space you might put a label to the side of the box that says...
(Enter a Y of N) ... or something to that effect.

You might also put a ! in the format property of the textbox to assure a capital Y or N. If you do not use that you should change the valid code to look for both upper and lower case Y or N.

Hope this helps.