Attribute a value to a record

Hello,

I'm currently running on pg8.4 and I have a trigger with a loop :

FOR ventilation_local IN (SELECT * FROM XXX) LOOP
    IF (mytest) THEN
        ventilation_local.myfield:=mynewvalue;
    END IF;
END LOOP;

my problem is that the record doen't accept the new value.
I've chek before the value that is not null.
Is it a fonctionnality accepted in pg8.4 on record type?

Best regards