[Igor_I]

Nitro-N, к примеру мой вариант.

Код:

                  <p> 
                    <input id='t' name="t" type="checkbox" value="on" onChange="getCheckboxforvisibility('t', 'сt') " />
                  </p>
                  <div id='ct' style="visibility: hidden">
                    <select  id='tsite'  name="tsite" size="1">
                      <option></option>
                      <option></option>
                      <option></option>
                      <option></option>
                    </select>
                  </div>

function getCheckboxforvisibility(getid, setid)
    {
    		if (document.getElementById(getid).checked == false)
      		{
   					document.getElementById(setid).style.visibility = "hidden";
         	};
          	if (document.getElementById(getid).checked == true)
           	{
   					document.getElementById(setid).style.visibility = "visible";
   			};
   	};