Hello, it works for me in this way:
Private Sub TxtMotivo_AfterUpdate()
Dim B As Integer
Select Case Me![TxtMotivo]
Case 1
Me![Tipointeres] = "NAT"
B = 1
Case 2
Me![Tipointeres] = "CONT"
B = 2
Case 3
Me![Tipointeres] = "CULT"
B = 3
End Select
DoCmd.RunCommand...