How do I populate a dropdown list based on a selection of another dropdownlist on the same page.

Trying to figure out how to populate one dropdown list based on the selection of another drop down list.  Back in the old days, I would have passed the user between two pages.

But, I have a list of Offices, based on a stored procedure that gets all the OfficeNames, and OfficeIDs.  I then have another drop down box of Employees, and based on which office you picked, I only want to display employees that work in that office.    How do I pass a value from the first drop down into the stored procedure to populate the second drop down.  (And, then because I know this might come up again, what if I have three drop downs, with Office, Department, and then employees)  How can I send two values to run stored procs each time to filter department and then employee?)

My vb.net code..

<asp: ...