Principal peaks in the mass spectra of deoxynupharidine are accounted for on the basis of high resolution mass spectrometry and peak shifts in the mass spectra of deoxynupharidine-4-d1 and deoxynupharidine-6/3,7,9-d2. Cleavage of the quinolizidit~e ring system of deoxynupharidine occurs predominantly in ring A, giving m/e 136, 107, 98, 94 and 81. The origins of the family of ions [Malkyl]+ and m/e 178 are considered. A comparative survey is made of the occurrence of these ions, and others, in the mass spectra of piperidine, quinolizidine and thiospirane types of Nuphar alkaloids. Attention is directed to ions useful in distinguishing structural types.MASS spectral data have been given for several Nuphar alkaloids in the reports of the structure elucidation of these compounds. However, not all of these reports have given evidence for the origin of the peaks, nor has there been a broad comparison of the spectra of various structural types of Nuphar alkaloids. We present here a comparison of the mass spectra we have obtained for several Nuphar alkaloids and indicate peaks that are useful in distinguishing the various structural types. We also call attention to the origin of principal peaks for those cases where the mass spectra of deuterium labeled compounds* and high resolution spectra are available.Fundamentally Nuphar alkaloids are of two structural types; those containing, respectively, piperidine (I to 111) and quinolizidine ring systems. The latter can be further subdivided into two groups; those having a simple quinolizidine ring system (IV to XVI) and those having two such systems (XVII to XXII). (-)-Deoxynupharidine (DON) (IV) is the parent C,,-quinolizidine type alkaloid whose carbonnitrogen skeleton is found in naturally occurring C,, functional group variations of DON (VII, VIII and X), and incorporated into the C,,-thiospiranes (XVII, XX and XXI) and the 6,7/3-oxidodeoxynupharidine dimer (XXII). Since the structure of DON is central to the class of quinolizidine alkaloids, we shall first discuss its mass spectral characteristics in view of labeling and high resolution data. Thereafter we will indicate the fate of each of the principal ions in the mass spectra of the more complex CI5 and C,,-alkaloids~ as well as pointing out the new peaks which appear.* The preparation of each of the deuterium labeled compounds is reported in the reference cited for that compound. t The mass spectral data for several epimeric alkaloids have been determined but since the low resolution mass spectra of epimers are very similar, we will only give the data for one of the epimers. An exception is the thiospirane alkaloids. Epimeric pairs whose mass spectra have been determined are: deoxynupharidine and 7-epideoxynupharidine, nupharolutine and 7-epinupharolutine, nuphamine and 3-epinuphamine, deoxynupharidin-6, 7a-diol and deoxynupharidin-6,7@-diol, 7@-methylthiodeoxynupharidin-6a-01 and 7a-methylthiodeoxynupharidin-6P-ol, 7,!7-methylthiodeoxynupharidine and 7cc-rnethylthiodeoxynupharidine, 6,7[j-oxidodeoxynupharidine dimer a...