basic SQL request

Hi,

i've tried to run a basic SQL request as followed :
select *
from articles, articletypes, department
where
    articles.articletype_id = articletype.articletype_id AND
    articles.department_id = department.department_id AND
    articles.validity_period_end > now()

and i got the following error message :
ERROR:  missing FROM-clause entry for table "articletype"

i'm confused now, if i use LEFTJOIN it's the same, so where is the trouble ?

thx,

AL.