<script type="text/javascript">   
function op(obj){   
if(document.getElementById){   
var el = document.getElementById(obj).style.display = "block";   
var ar = document.getElementById("masterdiv").getElementsByTagName("div");   
for (var i=0; i<ar.length; i++) { if (ar[i].className=="minfa") ar[i].style.display = "none";}   
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<a href="javascript://" onClick="op(i1); return false;">Оснавная инфа</a> <a href="javascript://" onClick="op(i2); return false;">Дополнительная инфа</a>   
<div id="masterdiv">   
<div id="i1" class="minfa">$BRIEF$</div>   
<div id="i2" style="display:none" class="minfa">$MESSAGE$</div>   
</div>