High resolution molecular beam maser spectroscopy has been extended into the submillimetre wave region of the spectrum. The maser spectrometer consists of a multiple beam source, stacked quadrupole state selector, and Fabry–Perot cavity. The submillimetre wave stimulating signal is produced by harmonic generation from a millimetre-wave klystron, directly referenced to WWVB. The maser signal is detected in a cryogenic photodetector. Linewidths 50–100 times narrower than the Doppler width and good signal-to-noise ratios were obtained. This system has been used to resolve the small deuterium hyperfine splittings of both the 110 → 101 transition of D2O at 317 GHz and the J = 1 → 0 transition of ND3 at 309 GHz. The relevance of the D2O quadrupole interaction data to the electric field gradient in water is discussed. The spectroscopic constants in kilohertz for D2O in the states 110 and 101 are, respectively, (eQqJ)D = 34.81 ± 0.32, CD = −2.661, and (eQqJ)D = −29.60 ± 0.55, CD = −2.532. The constants for ND3 are, in kilohertz: (eQqJ)N = 816.2 ± 1.1, CN = 2.0 ± 1.5, (eQqJ)D = 19.9 ± 0.8, and CD = −2.2 ± 0.8.