combo box

Question

my question is tht i hav two forms.

1.category form...tht hav 2 fields category_id and categtory_name

2.sub category form tht hav 3 fields.category_id,sub_category_id,sub_category_name


in 3rd form retrieve those value from category and sub category..

when i change value of category name combo box..tht time in sub category name also have to be change

how shuld i do....