Cortical neurons that are tuned to sound intensity (non-monotonic neurons) are very important for processing auditory information. Considering the fact that all auditory nerve fibers have monotonical responses, inhibition in the primary auditory cortex (AI) is essential for intensity tuning. By using free field sound stimulation and in vivo extracellular recording, the present study investigated the intensity-tuning properties in AI neurons of mouse (Mus musculus, Km). We also examined the effect of cortical application of the GABAa receptor antagonist bicuculline on AI intensity tuning in order to indentify the possible source of inhibition. The intensity-tuning curves were recorded in 72 AI neurons among which 28 showed monotonic responses and 44 showed non-monotonic responses. In non-monotonic neurons, there was no change in the first spike latency but a decrease in the firing duration (P < 0.01) when the sound intensity increased from best intensity to highest intensity. After AI application of bicuculline by electrophoresis (n = 28), the non-monotonic responses did not change in 39.3% neurons, was weakened in 42.9% neurons and was enhanced in 17.9% neurons. It suggested that GABAergic inhibition must be involved in AI intensity tuning but was not the only source. Non-monotonic excitatory input and non-GABAergic inhibition should also be considered. A postulated model was presented to explain the mechanism of intensity-tuning in AI neurons.