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tokens: the result of different masers flaring as the shock passes through the cluster, then we can estimate the distance between the flaring masers. If we assume the shock front is travelling at v ≈ 100 km s −1 = 10 7 cm s −1 and the time between flares is Δ T ≈ 1.5 × 10 7 s, we estimate the distance between the masers is Δ r ≈ 1.5 × 10 14 cm ≈ 11 AU. As already mentioned, the velocity extent of the water masers associated with W49N is ±200 km s −1 . Heckman Sullivan ( 1976 ) suggest that less than 10 per cent of the total integrated flux density across the entire spectrum is from these extreme-velocity masers. Also Goss et al. ( 1976 ) reported that the extreme-velocity maser features associated with W49N are weak relative to the low-velocity features. From the results of Very-long-baseline interferometry (VLBI) observations it has been inferred that the emission regions, where these extreme-velocity maser features exist, are more compact than those with intermediate (±20 km s −1 ) and velocities nearer to the local standard of rest velocity. The size of these maser clusters is less than 10 4 AU and they are located on the periphery of the intermediate-velocity maser features (Knowles et al. 1974 ). However, here we find that most of the water-maser flux density appears concentrated in the blue-shifted features and, in particular, the water maser at v = −81 km s −1 (Figs 2 and 3 ) is much stronger than the intermediate- and low-velocity masers. The extreme-velocity water masers associated with W49N have velocities that are more than 10 times faster than the isothermal speed of sound, ∼10 km s −1 , and concentrated in a region of only 0.1 pc in size. Larson ( 1973 ) showed that the accretion of matter onto a protostar has a velocity limited to a few times the speed of sound. Ergo, accretion cannot explain the appearance of extreme-velocity masers in W49N. Objects travelling at such extreme velocities would be dynamically unstable (Strel’nitskii Syunyaev 1972 ). Strel’nitskii Syunyaev ( 1972 ) proposed three possible acceleration mechanisms that may explain the extreme-velocity masers: acceleration by radiation pressure, stellar wind activity from hot massive stars, and by shock waves propagating through the maser space. Silk Burke ( 1974 ) suggested another acceleration mechanism: shock waves from supernova explosions. There is a supernova remnant near W49B; however, there is no non-thermal source in the vicinity of W49N. This may not eliminate it as a possible explanation, since supernovae less than 100 years old do not produce distinguishable radio emission (Spencer Burke 1973 ). Low-mass protostars in the T Tauri phase (Strom, Grasdalen Strom 1974 ) have extreme-velocity optical spectral features (∼100 km s −1 ) known as Herbig–Haro objects (Kuhi 1964 ). However, this model (Herbig 1974 ) cannot explain the asymmetrical nature of extreme-velocity water-maser features associated with W49N. Asymmetric models associated with the scattering of light