Mammalian visual system develops after birth. In rats, postnatal 3 to 5 weeks are critical for their development (namely, critical period), during which the excitatory and inhibitory synapses mature to form functional circuits in visual cortex. Extracelluar single-unit recording techniques were used in vivo to investigate the difference of response characteristics of between neurons in young and adult rats' primary visual cortexes. It was found that: (1) The adaptation to sustaining flashing stimuli is significantly higher in neurons of young rats; (2) The evoked response rate of young visual cortex neurons is significantly lower than that of adult rats; (3) The spontaneous rate (without flashing stimuli) is significantly higher in young instead of adult rats; (4) Signal to noise ratio (SNR) is significantly lower in young visual cortex. These results indicate that the response capability to continued stimuli and the detectability is lower in developing visual cortex. It might result from the sequential development of excitatory and inhibitory synapses in postnatal rat neocortex.