oracle not in query takes longer than in query -

i have 2 tables each has 230000 records. when make query:

select count(*)  table1  field1 in (select field2 table2).

it takes 0.2 second.

if use same query changing in not in

select count(*)  table1  field1 not in (select field2 table2).

it never ends.

why ?

its better user not exists, not in uses row search takes long

GHI