Thahks I was searching on the Web and found this solution and and the problem was fix. This is solution using DAO:

Dim db As dao.Database

Dim tdf As dao.TableDef

Dim clave As dao.Index

Set db = CurrentDb()

Set tdf = db.TableDefs(T_Actual)

For Each clave In tdf.indexes

If clave.Primary = True Then

Campo_Actual = clave.Fields(0).Name

End If

Next

And this loop go fast.

Greetings.

Arsenio.