<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";}    
}    
}    
</script>    
<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>