How do I replace text inside a div element?

I need to set the text within a DIV element dynamically. What is the best, browser safe approach? I have prototypejs and scriptaculous available.

<div id="panel">
  <div id="field_name">TEXT GOES HERE</div>
</div>

Here’s what the function will look like:

function showPanel(fieldName) {
  var fieldNameElement = document.getElementById('field_name');
  //Make replacement here
}