In an interpretation of interstellar, circumstellar, and solar system CNO isotope ratios, we find two scenarios which are free of internal inconsistencies. The first requires that the early solar system was enriched by material from massive stars, leading to enhanced 12C/13C and 18O/17O ratios and to a reduced 14N/15N ratio. The second involves infall of gas onto the galactic disk after the formation of the solar system. Both scenarios require that the bulk of the interstellar 16O, 18O and 15N originates from massive stars (>8M⊙), with 18O and perhaps 15N being destroyed in lower mass stars. 17O is mainly synthesized in stars of intermediate mass while 12C, 13C, and 14N are produced in stars of high and intermediate masses.