Retinal bipolar cells play a key role in the processing of the visual signal because they are the only neurons transmitting information directly from photoreceptors providing the input to ganglion cells, which deliver the output. The ON class of bipolar cells are depolarized by an increase in light intensity, while OFF cells are hyperpolarized. Bipolar cells also contribute to the temporal filtering of the visual input into sustained and transient channels responding preferentially to slow or fast changes in light intensity (Masland, 2001). It is still unclear how the ribbon-type synapses of bipolar cells transmit voltage signals of opposite polarities and different kinetics. To monitor synaptic activity of bipolar cells, operating within the intact retinal circuit, we have made transgenic zebrafish expressing a new genetically-encoded reporter of synaptic activity. This reporter, SyGCaMP2, is composed of the calcium-sensitive fluorescent construct GCaMP2 (Tallini, 2006) fused to the synaptic vesicle protein synaptophysin. SyGCaMP2 was put under control of a promoter driving expression in ribbon synapses and calcium signals monitored across tens to hundreds of terminals in the inner retina by multiphoton microscopy. Zebrafish 9-12 days post-fertilization were anaesthetised by immersion in 0.016% MS222, immobilized in low melting agarose and injected with α-bungarotoxin (2mg/ml). Both spontaneous and light-driven activity were monitored through all the strata of the inner plexiform layer at rates up to 200 Hz and across one stratum at 1 kHz. We observed two basic types of presynaptic calcium signal in response to light: slow, sustained changes and faster calcium “spikes”. Sustained increases or decreases in calcium were observed in ON or OFF cells respectively, and were maintained throughout exposure to a step of light. These slow calcium signals are expected to modulate the rate at which vesicles are cycled through rounds of exocytosis and endocytosis (Lagnado et al., 1996). Calcium “spikes” were observed in both ON and OFF cells, rising to a peak in about 50 ms and declining in seconds. These fast signals are likely to reflect regenerative electrical activity through L-type calcium channels in the synaptic terminal (Burrone and Lagnado, 1997), which will trigger fast, transient, modes of exocytosis (Burrone, 2000). Individual bipolar cells transmitting in different strata of the IPL did not necessarily generate presynaptic calcium signals with the same kinetics: sustained signals were observed in some terminals and transient signals in others. These observations suggest that the basic unit for the generation of transient signals from the bipolar cells is the synaptic terminal rather than the whole neuron.