AbstractIntracellular recordings were obtained from supragranular neurons in slices of the rat visual cortex. In ∼25% of the cells large (0.5–1.6 mV) excitatory postsynaptic potentials (EPSPs) of constant amplitude were observed after minimal, presumably single‐fibre stimulation. The amplitude variance of these large EPSPs was surprisingly small and within the range of the variance of the noise. These EPSPs could be reduced in amplitude by paired‐pulse and low‐frequency stimulation or by raising extracellular Mg2+ concentration. Reduced EPSPs could either continue to behave as all‐or‐none responses, or they could fluctuate between several amplitude levels. Conversely, responses where the amplitude fluctuated from trial to trial under control conditions could be converted into large all‐or‐none responses by paired‐pulse facilitation. This indicates that the large all‐or‐none EPSPs were composed of several subunits, probably reflecting the action of several different release sites. It is concluded that these release sites are either independent and operate with a probability close to 1 or, if operating with a lower probability, are coordinated by a mechanism which synchronizes release. Several observations suggest that release probabilities can switch from values close to 1 to 0 with repetitive stimulation or high Mg2+ concentration. Thus, a substantial fraction of single‐fibre inputs to supragranular cells possess synapses which operate with high synaptic efficiency and extremely low variance under control conditions but can undergo drastic changes in efficacy when release probabilities are interfered with. Such modifications of release probability could serve as an effective mechanism to regulate the gain of synaptic transmission.