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Functional degradation of mammalian visual cortex is associated with aging. It has been hypothesized that much of the decline might be mediated by a degradation of cortical inhibitory system during senescence. In the present work, we compared the properties of adaptation, onset latency and signal-to-noise ratio in primary visual cortex of young and old rats using extracellular single-unit techniques. The short-term synaptic plasticity of young and old rats was also studied using field potential recording techniques. We found significant increased adaptation, prolonged onset latency, lower signal-to-noise ratio and decreased short-term synaptic plasticity in aged rats. The results are in accordance with previously reported functional declines in old monkeys and old cats, indicating a universal mechanism of degradation in cortical function that accompanies old age in different mammalian species.