The use of increased sensitivity and temporal resolution at mm-wavelengths has revealed a number of novel features associated with solar activity (Kaufmann et al., 1979a). Of particular interest are the ultra-fast time structures superimposed on impulsive bursts. They are distinguished by time scales and repetition rates much shorter than the impulsive times scales. For weak bursts at λ = 13mm (i.e., few s.f.u.) the superimposed spikes are separated in time by several seconds. For moderate fluxes (i.e., 100 s.f.u.) the repetition rate of superimposed spikes is ~ 10–15 per second (i.e., time interval between two repeated spikes ~ 80 msec). For bursts larger than 500 s.f.u. the predicted times between repeated ultra-fast structures are shorter than 10 msec. Various examples have been studied with a time resolution better than 10 msec. (Kaufmann et al., 1979b). The impulsive flux level increases with the repetition rate of the ultra-fast component, following a nearly linear relationship. The simplest interpretation suggests that the injected spikes are associated with the bursting source function and are quasi-quantized in energy (Kaufmann et al., 1979b).