Interaction of energetic electrons (16–300 keV) and ELF waves (100 Hz–3 kHz) has been studied by using data of the GEOS-1 and -2 satellites. In steady-state, it is demonstrated that this interaction enters quite well into the framework of the quasi-linear theory of pitch-angle diffusion. By calculating the wave growth rate at different magnetic latitudes, it is shown that inside the plasmasphere the path integrated gain is large enough for explaining generation of hiss, in agreement with theoretical and experimental results on wave propagation. Outside the plasmasphere hiss generation deserves further study. At the time of storm sudden commencements, non-steady state behaviour is observed that results in antiphase oscillations of the maximum of the wave spectrum and of the corresponding growth rate and anisotropy.