Code:

uses

  RichEdit;

procedure RE_SetLineSpacing(ARichEdit: TRichEdit; lineSpacing: Byte);

var

  pf2: ParaFormat2;

begin

  FillChar(pf2, SizeOf(pf2), 0);

  pf2.cbSize := SizeOf(PARAFORMAT2);

  pf2.dwMask := PFM_LINESPACING;

  pf2.bLineSpacingRule := lineSpacing;

  SendMessage(ARichEdit.Handle, EM_SETPARAFORMAT, 0, Longint(@pf2));

end;

//Example: Setlinespacing to 1:

procedure TForm1.Button1Click(Sender: TObject);

begin

  RE_SetLineSpacing(RichEdit1, 1);

end;