You can use data validation to prevent typing in both cells. For
example, if the cells are C2 and D2:

Select cell C2, and choose Data Validation
From the first dropdown, choose Custom
In the Formula box, type: =D2=""
Select the Error Alert tab, and type your message.
Click OK

Do the same in cell D2, with the formula: =C2=""

Note that users would be able to paste values into both cells -- data
validation only prevents invalid entries that are typed.

Rob wrote:
I have created a time sheet with four time periods, there are two possible
entries in each period "Travel Time" and "Labor". I want to restrict entry to
one or the other of these cells per period. I would also like to display a
message if an attempt is made to enter data into both cells of the same
period. I have been unable to find a solution in the help files.
Thanks
Rob


--
Debra Dalgleish
Excel FAQ, Tips & Book List
http://www.contextures.com/tiptech.html