The rodent superior colliculus (SC) is a major target of retinal axons. Synaptic plasticity in the form of long-term depression (LTD) and long-term potentiation (LTP) is known to occur in the rat retino-collicular pathway, and this is known to change with age during early postnatal development (Lo & Mize, 2002; Mize & Salt, 2004), with a peak in LTD expression occurring at about postnatal day 9 (P9). The aim of the present study was to investigate these processes in the pigmented mouse. Parasagital slices (350μm) of the superior colliculus were prepared from C57BL/6 mice (age P8 to 10weeks) as previously described for the rat (Mize & Salt, 2004). Recordings of field excitatory postsynaptic potential (fEPSP) responses to submaximal stimulation of the optic tract (OT) were made in the superficial gray layer of the SC. After a stable control period (at least 15 min) of responses to pairs of test stimuli (0.1ms pulses, 20ms separation) repeated at 30s intervals, a 50Hz 20s tetanus was applied, following which test stimulation was resumed and responses were recorded for a further 60 min (Mize & Salt, 2004). In all experiments, the tetanus resulted in a period of short-term depression. In slices from young mice this was followed by a period of LTD (reduction of responses to less than 95% of control values). In P8-P13 mice (n=5), fEPSPs were reduced to 76±8.1% (mean ± standard error of mean, n=6 slices) of control amplitude 50-60 minutes after tetanus, and in P14-P17 mice (n=3) they were reduced to 82±5.7% of control (n=5 slices). During LTD, there was also a reduction in paired-pulse depression or increase in paired-pulse potentiation in slices from young mice. In contrast, in SC slices from adults (age 5-10 weeks, 7 mice) the effects of tetanus were less uniform, and either LTD (78±8.7% of control, n=3 slices), LTP (increase of responses to more than 105% of control values) (125±6% of control, n=5 slices), or no effect (102±1.7% of control, n=3 slices) was observed. Overall, in adult mice fEPSPs were 106±7.1% (n=11 slices) of control amplitude 50-60 minutes after tetanus. These findings indicate that there is a period of synaptic plasticity in young mice that manifests itself as LTD. This has similar characteristics to the LTD described previously in the young rat (Lo & Mize, 2002; Mize & Salt, 2004), and this coincides with a period of refinement of the retino-collicular pathway. In contrast, in adult mice a consistent LTD was not seen. This suggests that mechanisms of synaptic plasticity in adult mouse SC differ from those in developing mice.