Cajal suggested a century ago that Purkinje cell (PC) collaterals make synaptic contacts with other Purkinje cells (1). However, direct demonstration of a functional connection between Purkinje cells has been lacking. We have used two transgenic mouse lines that express GFP in PCs to facilitate the anatomical and electrophysiological study of these connections. Sagittal slices (200 – 300 µm) were prepared from P4–P15 mouse cerebellar vermis and slices were maintained at 32-34°C. Two-photon laser scanning microscopy was used to visualize putative presynaptic PCs expressing GFP, and to follow their axon collaterals in order to target putative postsynaptic PCs. Simultaneous triple or quadruple somatic recordings were made from putative pre- and postsynaptic neurons in current clamp (I-clamp, K⁺-based internal solution) or voltage clamp (V-clamp; Cs⁺-based internal solution). Since the Cl^– reversal potential is known to hyperpolarize over the developmental period studied here (2), symmetrical Cl^– internal solution was chosen. Neurons were hyperpolarized to ~-65 mV to prevent spontaneous discharge (or voltage-clamped), and presynaptic spikes were elicited by brief current injection. Monosynaptic connections were identified in 14% of attempts. PC-PC connections were shown to be GABAergic, as postsynaptic responses were blocked completely and reversibly by the GABA_A antagonist SR95531 (n=5). The average peak amplitude of the connections was 2.8 ± 1.2 mV (n=6), or 92.4 ± 16.1 pA (n=5) for I-Clamp and V-clamp recordings, respectively. The 20-80% rise time of the postsynaptic response was 3.9 ± 0.4 ms for I-clamp, and 0.78 ± 0.18 ms for V-clamp recordings. High-frequency presynaptic spike-trains between 10-100 Hz revealed strongly depressing synapses. Neurons were filled with biocytin for anatomical reconstructions using light microscopy. Putative synaptic contacts were predominantly somatic, with an average of 13.2 ± 2.5 putative synaptic contacts per connection (n=5). Since PCs are the sole output of the cerebellar cortex, the existence of monosynaptic PC-PC connections is likely to have important computational implications for cerebellar function, and may play a role in gain control and synchronization of the cerebellar circuit (3).