Release probability (P) appears to be a major factor that influences the pattern of transmitter release. At cortical pyramidal axon inputs onto different classes of target cells, very different release patterns are observed, patterns that correlate with release probability. Simplistically, 'low P' synapses display facilitation and augmentation, whereas 'high P' synapses supplied by the same axon exhibit paired-pulse and frequency-dependent depression. Different combinations of factors probably contribute to release probability at different terminals, during development and under different experimental conditions. The recent advances made by molecular biological studies of the release machinery do, however, provide candidate proteins and protein-protein interactions whose differential distributions might be important factors in determining the patterns of transmitter release.