1. While the presence of autapses in the brain is indicated by a large body of morphological evidence, the functional role of these structures has remained unclear. To probe for autaptic currents, we have recorded current responses following short somatic depolarizing pulses in Cl--loaded interneurones (stellate and basket cells) from rat cerebellar slices (animals aged 27-39 days). 2. In approximately 20 % of the recordings, fluctuating inward current transients were obtained with a latency of 1.15-2.45 ms (measured from the peak of the depolarization-induced Na+ current; n = 10). 3. These transients were blocked by bicuculline and were sensitive to the extracellular Ca2+ concentration. 4. Assuming low release probability, as suggested by the high failure rate (0.65-0.92, n = 10), quantal sizes ranging from 21 to 178 pA (-70 mV; n = 10) were calculated from a variance analysis of autaptic current amplitudes. 5. We conclude that approximately 20 % of interneurones have a functional autapse. Autaptic currents may inhibit firing of interneurones during high frequency discharges.