At the Voyager 1 spacecraft in the outer heliosphere the intensities of both anomalous cosmic rays ( ACR ) and galactic cosmic rays ( GCR ) changed suddenly and decisively on August 2 5 th ( 121.7 AU from the Sun ). Within a matter of a few days, the intensity of 1.9 – 2.7 MeV protons and helium nuclei had decreased to less than 0.1 of their previous value and eventually the intensities decreased by factors of at least 300 – 500. Also on August 2 5 th the GCR protons, helium and electrons increased suddenly in just 2 or 3 days by factors of up to two . T he intensities of the GCR nuclei of all energies from 2 to 400 MeV then remained essentially constant with intensity levels and spectra that may represent the local GCR. The suddenness of these intensity changes indi cate that V1 has crossed a well – defined boundary for energetic particles at this time possibly related to the heliopause.