DBMS_OUTPUT.PUT_LINE doesn't work

I try to have some text printed in sqlplus. I type:

exec DBMS_OUTPUT.enable;
exec DBMS_OUTPUT.PUT_LINE('hallo');
 I get the answer "PL/SQL procedure successfully completed."  for both commnads but nothing is printed. What's wrong?

Thanks in advance

--
Vagelis S. Hristidis
Electrical and Computer Engineering
National Technical University of Athens
e-mail:exrist_at_cc.ece.ntua.gr
URL:http://www.cc.ece.ntua.gr/~exrist