<form method="post" action="example.php" id="example" name="example">
<p>
<label>Список 1</label>
<select name="s" id="select1">
<option value="0">нет</option>
<option value="1">да</option>
</select>
</p>
<p id="p_select2" style="display: none;">
<label>Список 2</label>
<select name="s2" id="select2">
<option value="0">0</option>
<option value="1">1</option>
<option value="3">3</option>
</select>
</p>
</form>