Data from the Plasma Composition Experiment on ISSE 1, covering the energy range 0.1--16 keV/e, show that a dramatic change took place in the plasma sheet ion composition in conjunction with the magnetic substorm activity on March 22, 1979. Beginning about 1124 UT the ion population at the ISEE 1 location changed from what appeared to be predominantly ions from the solar wind to a mixture of comparable numbers of solar wind and terrestrial ions. ISEE 1 was inbound in the predawn sector during this time, and the plasma composition experiment provided data from R≂21 RE and LT≂0130, down to R≂3 RE and LT≂0530. Prior to the substorm activity about 90--95% of the ion density was due to H+ and He++ ions, which appeared to be mostly of solar wind origin. The H+ and He++ components, each approximated by a Maxwell-Boltzmann distribution, had a temperature ratio T(He++)/T(H+)≂4 and a density ratio n(He++)/n(H+)≂1.5--3%. Both values are consistent with measurements made concurrently in the solar wind by the plasma experiment on ISSE 3. The remaining 5--10% of the density was due mainly to O+ and He+ ions of ionospheric origin. All four ion populations had broad energy spectra with mean energies of several keV/e. About 25--30 min after the onset of the first substorm expansion at 1054 UT the H+ and He++ densities dropped sharply at ISSE 1 (at about 15 RE), and within the next 10 min the O+ density increased by almost an order of magnitude throughout the measured energy range, resulting in comparable amounts of O+ and H+. The O+ component of the plasma remained large or dominant through the remainder of the inbound pass. A significant portion of the O+ ions was contained within strongly collimated beams flowing tailward with energies mostly below 1 keV but occasionally reaching 6--7 keV. Similar beams of weaker intensity were also seen in the H+ and He+ components. The anticorrelation of ions from the two sources indicates that the particle flow pattern underwent a fundamental change in conjunction with thefirst substorm, possibly associated with a diversion of cross-tail currents through the polar ionosphere. The strong increase in the O+ density may suggest that the O+ ions carried a substantial portion of an outward field-aligned current connecting the auroral electrojet regions to the plasma sheet near ISEE 1 and beyond. The fact the O+ and He+ ions had energies much higher than what is typical in the ionosphere can perhaps be explained in general terms by a large electric ''potential difference'' along the magnetic field, but the broad energy spectra and varying pitch angle distributions of these ions indicate that the acceleration process was also ''diffusive'' in nature.