Public Class frmTest
 
    Dim rtf As New RichTextBox With {.Parent = Me}
 
    Dim WithEvents tim As New Timer _
        With {.Interval = 1000, .Enabled = False}
 
 
    Private Sub frmTest_Load(sender As System.Object, e As System.EventArgs) _
      Handles MyBase.Load
 
        rtf.Text = "AAAA " & vbCrLf & vbCrLf & "BBBBBB" & vbCrLf & _
          "CCCCCCCCCCCC"
 
        tim.Enabled = True 'Timer kann im Button-Click-Handler aktiviert werden
 
    End Sub
 
 
    Private Sub tim_Tick(sender As Object, e As System.EventArgs) Handles _
      tim.Tick
 
        Static lineindex As Integer = -1  'Zeilenindex jeweils merken
 
        Dim linesCount As Integer = rtf.Lines.Length 'aktuelle Zeilenzahl
 
        lineindex += 1 'Zeilenindex hochsetzen
 
        If linesCount = lineindex Then lineindex = 0 'umlaufender Index
 
        'aktuelle Zeile markieren  
        rtf.SelectionStart = rtf.GetFirstCharIndexFromLine(lineindex)
        rtf.SelectionLength = rtf.Lines(lineindex).Length
 
    End Sub
End Class