Edit in JSFiddle

setInterval(slide1, 3 * 1000);

function slide1() {
    $("#div1").toggle('slow');
    $("#div2").toggle('slow');
}

<div id="div1">Contenido 1</div>
<div id="div2">Contenido 2</div>

#div1 {
 display: block;  
 background: yellow; 
}
#div2 {
 display: none;   
 background: green;    
}
#div1, #div2 {
 width: 200px;
 height: 100px;   
}