Setting Textbox To Readonly?

In my asp page, I have a dropdown box and 2 textbox. The dropdown has "select" as the first value. If value is select in the dropdown, i want to make the textbox as readonly, else it can be used for inputting.

I tried this:

Code:

function OnChange(dropdown)
{
var myindex = dropdown.selectedIndex;
var SelValue = dropdown.options[myindex].value;

[Code].....

But, I am not able to set to readonly. How to make textbox readonly on the select change and body onload?