Private Sub ComboBox1_SelectedIndexChanged(ByVal sender As System.Object, _
   ByVal e As System.EventArgs) Handles ComboBox1.SelectedIndexChanged
        Select Case ComboBox1.SelectedIndex
            Case 0
                Call Brandenburg()
            Case 1
                Call Berlin()

Sub Brandenburg()
        'Landkreise Brandenburg
        ComboBox2.Items.Clear()
        ComboBox2.Items.Add("Oberhavel")
        ComboBox2.Items.Add("Havelland")
        ComboBox2.Items.Add("Barnim")
    End Sub