Related selects w/ defaults from session

I have a series of 3 or 4 related select boxes (contents of 2nd
dependent on selection in 1st,
contents of 3rd dependent on selection in 2nd and so on). I’m
implementing it using observe_field
on each select box and it works fine.

I want to remember what each user selects and pre-select an item in
each pulldown based on
his/her session. I’m storing the session data and can pre-select an
item in the first select box.
The second select box does not populate however, because there’s no
event fired from the
first select box.

How can I manually fire an event from the first select box which
triggers the population of the
second box?

Thanks,
Peter