1. A total of twelve synaptic connections between pairs of pyramidal neurones in layer 2/3 of slices of rat visual cortex maintained in vitro was investigated using whole-cell voltage recordings under visual control. The connections varied widely in strength, with the mean peak amplitudes of the resulting excitatory postsynaptic potentials (EPSPs) ranging between approximately 40 microV and 2 mV at 23 degrees C. The smaller mean amplitudes included a substantial proportion of apparent failures of transmission. 2. The properties of these EPSPs were examined over a range of temperatures between 13 and 36 degrees C. All the connections became more reliable, in that they showed fewer apparent failures of transmission, and showed less trial-to-trial variability at the higher temperatures. These changes appeared to be due primarily to an increase in the mean number of transmitter quanta released per presynaptic action potential. 3. At 36 degrees C most connections were relatively reliable, with a mean failure rate of only 16 %. Five connections showed virtually no failures (1 % or fewer) at this temperature. 4. We conclude that quantal transmitter release is temperature dependent at these synapses, and that experiments performed at room temperature could lead to an exaggerated impression of the unreliability of transmission at central excitatory synapses.