The neurochemical and behavioural recovery following unilateral and bilateral lesions of the nucleus basalis was investigated in adult male Wistar rats 20 days and 6 months after surgery. The lesions were made by stereotaxic injections of ibotenic acid. Twenty days after surgery there was a statistically significant choline acetyltransferase decrease in the frontal and parietal ipsilateral cortex of the unilaterally lesioned rats and in the cortex of both hemispheres after bilateral lesions. Cortical high affinity choline uptake rate was significantly decreased 4 days after lesions but showed a rapid recovery within 20 days post-lesion in unilaterally and bilaterally lesioned rats. However, at this time both groups of lesioned rats showed a marked impairment in the acquisition of passive and active (shuttle-box) avoidance conditioned responses. Six months after surgery the decrease in choline acetyltransferase activity was smaller and statistically significant in the ipsilateral frontal cortex only in the unilaterally lesioned rats and in the frontal and parietal cortex of both hemispheres in the bilaterally lesioned rats. High affinity choline uptake was increased in the contralateral hemispheres of the unilaterally lesioned rats and was significantly larger than in the bilaterally lesioned rats. There was no difference in the acquisition of both passive and active avoidance conditioned responses between the sham operated and unilaterally lesioned rats, while the bilaterally lesioned rats could only negotiate the active avoidance conditioned response. In conclusion, our experiments demonstrate a remarkable neurochemical and behavioural recovery within 6 months in rats with a unilateral lesion of the nucleus basalis and only a limited recovery in the bilateral lesioned rats.