1. Unitary inhibitory synaptic connections among stellate cells were studied in rat cerebellar slices. Presynaptic action potentials and inhibitory postsynaptic currents (IPSCs) were simultaneously recorded by loose cell-attached and tight-seal whole-cell recording, respectively. 2. Several types of synaptic connections were distinguished on the basis of the shape of the amplitude distribution of successfully evoked currents. For simple synapses, which presumably arise at single release sites, these histograms could be fitted to a single Gaussian (5 cases). In four additional cases a small amplitude component (< 50 pA) was superimposed to a single Gaussian peak. The small events had slow rise times and widely distributed amplitudes. Finally eleven histograms showed two or more Gaussian components and were classified as complex connections. 3. Failure rates ranged from 0.06 to 0.85 for unitary connections (n = 20) and from 0.59 to 0.78 for simple synapses (n = 5). 4. Coefficient of variation values derived from Gaussian fits to simple synapse histograms ranged between 0.20 and 0.38 (n = 5). 5. In simple synapses peak current amplitudes were positively correlated to both current rise time and decay half-width. 6. Intervals between presynaptic action potentials were widely distributed. During stationary periods there was no correlation between interspike interval and amplitude size, success rate or latency. In some experiments, episodes with shorter interspike intervals were observed. During these periods, amplitude and success rate decreased, and the latency increased. Thus, IPSC characteristics depend on the mean frequency of presynaptic spikes, but not on random fluctuations of interspike intervals during stationary periods.