Miniature postsynaptic currents (minis) in cultured retinal amacrine cells, as in other central neurons, show large variations in amplitude. To understand the origin of this variability, we have exploited a novel form of synapse in which pre- and postsynaptic receptors sample the same quantum of transmitter. At these synapses, mini amplitudes measured simultaneously in the 2 cells show a strong correlation, accounting for, on average, more than half of the variance in amplitude. Two pieces of evidence support the conclusion that variations in the amount of transmitter in different quanta underlie this correlation. First, diazepam, which enhances GABA binding, increases mini amplitude, implying therefore that transmitter concentration is not saturating. Second, we show that amplitude distributions from all cells, even those with a small number of release sites, have the same shape, implying that most or all variance is intrinsic to each release site.