<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<style>
#second {
display: none;
}
</style>
</head>
<body>
<select id="first">
<option value="0" selected >Не выбрано</option>
<option value="1">Авто\Мото</option>
<option value="2">Компьютеры</option>
<option value="3">Знакомство</option>
</select>
<select id="second"></select>
<script>
var autos = 'BMW X5, Jaguar F5, Lamborgini Italian'.split(', ');
var computers = 'Windows, Mac, Linux'.split(', ');
var connexions = 'Love, Meeting, Sex'.split(', ');
var two = [autos,computers,connexions]
document.getElementById('first').onchange = function(){
var select = document.getElementById('second'),
j = this.value;
select.style.display = "none";
select.options.length = 0;
if (j) {
var options = two[--j];
for (var i = 0; i < options.length; i++) {
select.options[i] = new Option(options[i], i); ;
}
select.style.display = "inline-block";
}
}
</script>
</body>
</html>