Rats were trained for fifteen sessions in an automated eight arm radial maze prior to treatment with 6 mg/kg trimethyltin chloride. This compound is a neurotoxicant which primarily damages the limbic system, in particular pyramidal cells in the CA3 region of the hippocampus. Following treatment the animals exhibited a marked and persistent impairment of maze performance characterized by decreased selection accuracy and an altered spatial pattern of responding within the maze. These results offered additional evidence that CA3 pyramidal neurons or their connections play an important, if not essential, role in radial-arm maze performance. It was suggested that trimethyltin might be a useful tool for elucidating the neural substrates of both radial maze performance and learning and memory processes.