The relaxation of the isothermal dc magnetization M in zero-field-cooled Ba-La-Cu-O and Sr-La-Cu-O has been investigated. After the external magnetic field has been changed or cycled, the resultant magnetization M decays following the law Mln(tt0) with great accuracy over an observation time period of four decades. At H=0, the logarithmic decay rate Mlnt is proportional to the temperature T and it is also proportional to Hi3. Here Hi is the external field which is reduced to zero before the decay rate of persistent currents is observed. © 1987 The American Physical Society.