How can I set the value of a DropDownList using jQuery?

$("#mydropdownlist").val("thevalue");

just make sure the value in the options tags matches the value in the val method.

As suggested by @Nick Berardi, if your changed value is not reflected on the UI front end, try:

$("#mydropdownlist").val("thevalue").change();

If you working with index you can set the selected index directly with .attr():

$("#mydropdownlist").attr('selectedIndex', 0);

This will set it to the first value in the droplist.

Edit: The way I did it above used to work. But it seems like it doesn't any longer.

But as Han so pleasantly points out in the comments, the correct way to do it is:

$("#mydropdownlist").get(0).selectedIndex = index_here;