Spontaneous electrical activity in the early visual system is thought to regulate a number of processes that lead to the establishment of appropriate synaptic connections during development (Wong, 1999). This activity has been studied in detail in retinal ganglion cells, and is modulated as the retinal circuitry matures and ON and OFF bipolar cells establish connections with specific ganglion cells in the inner plexiform layer (IPL). The mechanisms that lead to spontaneous activity in retinal ganglion cells are still unclear. Furthermore, it is not known how far spontaneous activity in ganglion cells might be driven by glutamatergic synaptic inputs from bipolar cells. We have monitored synaptic activity of bipolar cells in the retina of live zebrafish (anaesthetised by immersion in 0.016% MS222) between 3 and 9 days post fertilization (dpf) using SyGCaMP2, a genetically encoded reporter of presynaptic calcium signals (see abstract by Dreosti et al.). Spontaneous activity was detected as early as 3 dpf and two different patterns were observed: slow calcium waves with a period of 30 – 120 s, and fast regenerative calcium “spikes” which rose in 0.5 – 0.8 s and decayed exponentially with a time-constant of 1.5 – 4.5 s. A given terminal displayed either fast spikes or slow waves in calcium, but not both. The frequency and pattern of calcium spikes varied widely between terminals, and occured at instantaneous frequencies up to 1 Hz. The percentage of terminals generating calcium spikes reached 77% (n=39) at 5 dpf, declining to 30% at 9 dpf (n=23). In contrast, slow calcium signals were most prevalent at 3 dpf (46 %; n=35), declining to 4% (n=23) at 9 dpf. In some terminals, uniform steps of light lasting 30 s caused either a steady increase or decrease in presynaptic calcium, identifying ON and OFF cells. The percentage of terminals displaying responses to light increased from about 6% at 3 dpf (n=35) to 65% at 9 dpf (n=23). About 8% of the terminals responsive to light (n=48) generated spontaneous calcium spikes, and none exhibited slow waves. Thus the development of responsivity to light was correlated with a decrease in spontaneous synaptic activity. These results suggest that synaptic transmission from bipolar cells is one of the mechanisms generating spontaneous spikes in ganglion cells over the period that the retinal circuit is developing.