Changes of acetylcholinesterase activity and its molecular forms, extracted by Triton X-100 and separated by polyacrylamide gel electrophoresis, were studied in the rat hippocampus following septal lesions. Detection of acetylcholinesterase was made densitometrically. While the total activity of acetylcholinesterase was decreased, its molecular forms exhibited a different pattern of changes: the heavy forms were decreased, while the light ones were increased. The results support the view that different acetylcholinesterase molecular forms serve different regulatory mechanisms.