Show div on selection from dropdown

This might be more of an HTML general question, but I have been trying
to find a "rails" way to do this and have come up short. What I want to
do is to have a div appear if a certain selection from a drop-down is
selected or dissapeare if another option is selected. Here is what I
have:

<select id = "user_role" name = "user[role]">
  <option value = "Admin" <%= @user.role == 'Admin' ? 'selected =
"selected"' :
    nil %>>Admin</option>
  <option value = "Agency Admin" <%= @user.role == 'Agency Admin' ?
    'selected = "selected"' : nil %>
    onselect = "alert('TEST');
    document.getElementById('agency_admin_div').style.display = '';">
    Agency Admin</option>
  <option value = "Agent" <%= @user.role == 'Agent' ?
    'selected = "selected"' : nil %>>Agent</option>
  <option value = "Travel Manager" <%= @user.role == 'Travel
    Manager' ? 'selected = "selected"' : nil %>>Travel Manager</option>
  <option value = "Traveler" <%= @user.role == 'Traveler' ?
    'selected = "selected"' : nil %>>Traveler</option>
</select>

This doesn't work, and I have been unable to find an oppropriate rails
or html example that gives me what I want. I have done similar things
with rails helpers before but never with the "select" tag. Anybody have
any suggestions? Thanks,

-S