The mass spectra of several isofuroquinoline alkaloids (N- methylfuroquinol-4-ones) including the new N-prenylfuroquinol-4-one, acrophylline, have been recorded. The fragmentation of hexahydroacrophylline and related 3-ethyl-4-hydroxyquinol-2-ones have also been determined by deuterium and oxygen-18 labelling and high resolution measurements. © 1969, CSIRO. All rights reserved.