plpgsql question

v8.3.4 on linux

Is there a way to set the query used in a "for rec in (query) loop -> end loop" be a variable?  Example

if (foo = 'whatever')

then

  sqlstmt := "select x,y,z ...";

else

  sqlstmt := "select a,b,c ...";

end if ;

for therec in

  sqlstmt

loop

    ...

end loop;

Thanks in Advance for any help.