Thanks for your help Jim,

With tests I made, I can see a difference modifying a lot of rows (like 200000) and a lot of fields on each row (like 25). In the case of multiple REPLACE by row, I can also see a difference between doing no RLOCK (this lets VFP do it for each REPLACE), and doing my own RLOCK then the 20 REPLACE than the UNLOCK... On usual tasks, the difference is (as you said) not very significative !

Thanks again to all !