<!DOCTYPE html>
<html>
<head>
<style>
select, div {
display: inline-block;
}
</style>
</head>
<body>
<select id="first">
<option value="0" selected disabled>Не выбрано</option>
<option value="1">Авто\Мото</option>
<option value="2">Компьютеры</option>
<option value="3">Знакомство</option>
</select>
<div id="second"></div>
<script>
var autos = 'BMW X5, Jaguar F5, Lamborgini Italian'.split(', ');
var computers = 'Windows, Mac, Linux'.split(', ');
var connexions = 'Love, Meeting, Sex'.split(', ');
document.getElementById('first').onchange = function(){
var str = '';
if (this.value == 1) {
for (var i = 0; i < autos.length; i++) {
str += '<option value="i">' + autos[i];
}
} else if (this.value == 2) {
for (var i = 0; i < computers.length; i++) {
str += '<option value="i">' + computers[i];
}
} else if (this.value == 3) {
for (var i = 0; i < connexions.length; i++) {
str += '<option value="i">' + connexions[i];
}
} else return false;
document.getElementById('second').innerHTML = '<select>' + str + '</select>';
}
</script>
</body>
</html>