Chronic morphine exposure results in degradation of the functional properties of cortical cells. However, little evidence has been reported about the effect of morphine on the temporal properties of the visual system. We compared the visual response latency of different cells of the lateral geniculate nucleus (LGN) in morphine (10 mg/mL)- and saline-treated cats. Morphine-treated cats were given morphine sulphate by cervical subcutaneous injection twice daily for 10 days, whereas saline-treated cats received injections of 0.9% saline instead. We found that LGN neurons in morphine-treated cats exhibited significantly longer response latency than those in saline-treated cats (P < 0.001). To investigate whether different types of neurons exhibited similar changes in response to morphine, we classified LGN neurons as on- and off-centre, X and Y, and layer A, A1 and C neurons. There was a tendency for prolonged latency in layer C neurons and a significantly longer latency for the other neurons in morphine-treated cats (P < 0.05). These findings suggest that chronic morphine administration (10 mg/kg by cervical subcutaneous injection twice daily for 10 days) delays information transfer earlier in the visual pathway.